Mintt favoritprogram är South Park.

If you want a grammatical explanation, here you go:

The noun ‘program’ (which is the base morpheme of the word) belongs to the grammatical gender ‘neutrum’.
Nouns in the gender neutrum (generally) end with /-t/ or /-et/ in the definite form, eg. ‘hus|et|’. Some pronouns that can be used with neutrum nouns are:
{mitt, ditt, sitt, vårt, ert}*

The other Swedish gender is ‘utrum’. Nouns in the gender utrum (generally) end with /-n/ or /-en/ in the definite form, eg. ‘bil|en|’. Some pronouns that can be used with utrum nouns are:
{min, din, sin, vår, er}*

Note that the pronouns used with utrum do *not* end with /-t/. Instead, the /-t/ is in some pronouns just removed, while in other it’s replaced with /-n/


* {my, your (sing.), her/his/its/their (sing.), our, your (pl.), respectively}

Glossary:
morpheme = the building blocks of a word. In Swedish, many words are compound words created by using regular words as morphemes to build a new word, eg. “nagel|lacks|bort¦tagnings|medel” or ‘nail|polish|removing|(not really a word for it, closest is maybe acid?)’ ie ‘nail polish remover’

(grammatical) gender = the ‘gender’ of words (often nouns) in a language. Some languages use feminine and masculine. Some use completely different genders. Swedish uses neutrum and utrum.


TL;DR:
Utrum = nouns ending with /-n/ or /-en/. —> eg. ”bil|en”, use ”min”
Neutrum = nouns ending with /-t/ or /-et/. —> eg. ”program¦m|et”, use ”mitt”

Det ähandlar om fyra pojkar -– Eric Cartman, Stan Marsh, Kyle Broflovski, och Kenny McCormick, och sinsamt deras vänner.

I removed the Oxford comma, because it does not always work very good in Swedish, such as in this case.
I also replaced the second ”och” with ”samt”. It kind of means ‘and also’. Using ”samt” looks so much better when writing—especially after several ”och” or commas. Moreover, it also sounds a bit more educated.

(You wrote ”och sin vän[n]er”. I *think* that you meant ‘and their friends’—in other words, that the friends were friends with all of said boys. I edited from that understanding, at least.)
I changed ”sin vän[n]er” to ”deras vänner”. The reason is that the friends are friends to all of said boys. In other words, they grammatically ‘belong’ to a plural number of people. Third person plural possessive pronoun is in Swedish ”deras”. (This is both when the object of the sentence is in singular and plural: ”deras boll” cf. ”deras bollar”. So in this sentence it would work with both ”deras vän” as well as ”deras vänner”. Note that this is not how it works with some of the other pronouns. One example is ”min vän” but ”mina vänner”.)

Eric Cartman är en tjock, snål, antisemitisk, och en manipulativ lögnare.

The sentence is in theory correct. *But* I still changed it. The reason for this is that it makes the sentence flow out more easily when spoken, and will make ”manipulativ” and ”lögnare” more connected in the understanding of the reader.
If you’d like to pronounce on the “manipulativ lögnare”, you could add ”dessutom” in front of ”en”. ”Dessutom” means ‘moreover’, and can be pronounced a bit harder to give a better effect.

Han ofta lura sin vän Butters Stotch- en snäll pojke - i tar hand om hemsk sakelurar ofta sin snälla vän Butters Stotch till att utföra hemska handlingar.

In Swedish, you can’t say ‘he often tricks [...]’ as you can in English. Instead, you say ”he tricks often [...]”. That is ”han lura¦r ofta [...]”. The verb ”att lura” (‘to trick [someone into something]’) is conjugated into ”[personal pronoun] lura¦r” in present tense. Do note that ”ofta” comes *after* ”lurar”.

Dashes are used in Swedish, but not to the same degree as in English—especially not in simple(-r) texts like this one. So instead of ‘[...] his friend Butters Stotch – a kind boy – [...]’, you say ‘[...] his kind friend Butters Stotch [...]’. You can do the same in English. I’d even advise you to do the same in English in short, simpler sentences.

”[...] i tar hand om [...]” could be written as ”[...] till att ta hand om [...]”, but it’s pretty long, don’t you think? (Though, it is correct, and an idiom. Nice :) ) Instead, we can use the verb “utföra” (‘perform’) in infinitive. The sentence becomes ”[...] till att utföra [...]” (‘into to perform’. With working English grammar: ‘into performing’)

”[...] hemsk saker [...]”. ”Saker” is plural (‘things’). Therefore, the adjective should also be plural: ”hemska”. I believe you are trying to use ”sak¦er” as the English ‘deed¦s’. That would in Swedish be ”handling¦ar”, so we’ll use that instead. ”Sak¦er” *is* correct and works in many, *many* cases, but in this case, ”handling¦ar” works better. Therefore, ”hemska handlingar”.

Han har en alkoholberoende pappa vems är också innerst, som egentligen är Lorde.

In this sentence, I think you’re confusing ‘who’s’ (‘who is’) and ‘whose’. ”Vems” means ‘whose’ in English, like in ‘Whose cap is this?’. What you really mean is ‘who’s’
‘Who is’ has two translations in Swedish: 1) ”Vem är [...]?” or ‘Who is [...]?’, like in ‘|Who is| the chef?’, and 2) ”Som är” or ‘[...] who is [...]’ [note: no question mark.], like in ‘I spoke to Sarah’s mom, |who is| principal in your school.’.
What you mean is number two – ‘som är’.

We could just write ”[...] som är Lorde.”, but then we miss the ”är också innerst”. Instead of that, you could say ”innerst inne är”, but I chose to change it to ”egentligen”. This keeps it more natural. (”Innerst inne” sounds a bit…like your soul is something? It *is* correct, but more often used in other situations.) Therefore, ”egentligen är”. (Note the change in word order; ”egentligen” is contained between ”som” and ”är”.

Moreover, I added a comma to keep reading easier.

Kyle Broflovski är en judisk pojke med skarpfasta principer, och den här är vt är därfoör Cartman hatar hanonom.

To keep your sentence as correct and intact at the same time as possible, I chose to keep the word order by writing ”med fasta principer”, instead of changing the word order and using ”principfast”, as another user suggested. Writing it my way is generally understood as Cartman hating Kyle because of his principles, while writing it as the other user suggested is understood as Cartman hating him either because of him being a jew, or him being a boy – depending on which word is pronounced hardest.
(Disclaimer: I have not seen South Park, so I do not know why Cartman hates Kyle. Because of this, I only guessed the meaning from the sentence, and I have no idea if my guess is the right one or not)

I replaced ”skarpa” with ”fasta”, as I feel like it’s the word that would’ve been used in a normal conversation. It means ‘steady’ or ‘solid’.

”[...] den här är värfor [...]” You’re trying to say ‘[...] this is why [...]’. In Swedish, there are two ways to say it: either ”[...] det är därför [...]”, or ”[...] detta är varför [...]”.
The first one is literally ‘[...] it is because [...]’. It does sound a bit weird when it’s placed in the sentence. But you must note that the ”det” or ‘it’ refers to the previous clause, so translated it is ‘[...] that is the reason why [...]’.
The second one is literally ‘[...] *this* [or *that*] is why [...]’.
In English, both of these would require the comma between the sentences to be a full stop.
”Varför” = ‘Why’, as in ‘Why do you never clean your room?’
”Därför” = ‘Because’, as in ‘Because it’s boring.’
Note that “varför” and ”därför” are not always used the way ‘why’ and ‘because’ is in English, such as in your example.

”Cartman hata han” ”Hata is the infinitive, while ”[han] hata¦r” is the conjugated form in present tense.
Writing ”han” instead of ”honom” is a mistake that is so very common. Even many (*many*) native speakers mistakenly use ”han” for both ”han” and ”honom”.
”Han” and ”hon” are the subjects of a sentence,
while ”honom” and ”henne” are the objects.

In easier words:
”Han” = ‘He’ ”Hon” = ‘She’
”Honom” = ‘Him’ “Henne” = ‘Her’

Therefore, ”[...] Cartman hatar honom.”.

Kenny McCormick är en fattig pojke vemssom bara har en orange jacka, vilken täcka sin mun på sigalltid täcker hans mun.

Like before, you confuse ‘who’s’ (‘who is’) and ‘whose’. The sentence should say ”som”, not ”vems”.

I changed ”vilken” to ”vilken alltid” (‘which always’) to make the clause have an actual meaning.

”Att täcka” is in present tense conjugated to ”[den] täck¦er”.

”Vilken” makes the orange jacket the subject of the second (or third, depending on how you see it). ”Sin jacka” does therefore in your sentence mean ‘the orange jacket’s mouth’. ”Sin” is a word not found in many languages, for example in English, which makes it hard for learners to understand its use.
”Sin” gives the reader the information that the word following it – in this case mouth – belongs to the subject – in this case the jacket.
What you want to write instead is ”hans”. It means ”his” and lets the reader know that the word following belongs to ‘he’, not the subject. Who ”han” (or any other pronoun, even ”sin”) is is understood through the context.
Therefore, ”hans mun”

I removed ”på sig” because that is already understood through the verb ”[att] täcka” / “täcker” or ‘[to] cover’ / ‘cover’.

Han dör i alla avsnittar.

This sentence is a bit tricky. ”Avsnitt” is an irregular word, and does not conjugate in plural. That means it’s both ”ett avsnitt” and ”flera avsnitt”.

Also, ”[att] dö” is in infinitive, use the conjugated ”dör” instead.

Det ähandlar om fyra pojkar - Eric Cartman, Stan Marsh, Kyle Broflovski, och Kenny McCormick, och sinderas vänner.

"Handla om" is the verb phrase for to "be about" something.

"Sin" is for something belonging to one person. "Deras" for something belong to more than one.

The Oxford comma isn't proper in Swedish

Han ofta lurlurar ofta sin vän Butters Stotch- en snäll pojke - i[?] tar hand om hemska saker.

V2 word order. "Ofta lurar han" would also work, it depends on where you want the emphasis.

Unsure what what intended in the last bit. It's not comprehensible as written, but it would be if the "i" was replaced by for example "och" (Eric takes care of horrible things) and "som" (Butters takes care of horrible things)

Han har en alkoholberoende pappa vems är också innerst Lordesom också är Lorde i hemlighet.

Assumed "innerst" ("innermost") was intended as an "in secret" of sorts.

"vems" –> "som" (see below)

Kyle Broflovski är en principfast judisk pojke med skarp principer, och den härt är vdärfoör Cartman hatar han.onom

Varför ~ "why?" (question, generally)
Därför ~ "therefore" (explanation)

In standard Swedish:
"Han" ~ "he"
"Honom" ~ "him"
(full disclosure: remnants of old accusative/dative peculiarities do exist dialectally – but you shouldn't worry about that)

Kenny McCormick är en fattig pojke vemssom bara har en orange jacka, vilken täcka sin mun på sig som täcker hans mun.

"vems" – "whose?" (question)
"vilken" – "which?" (question, generally)
"som" – "which"/"who"/"that" (explanation)

Han dör i alla avsnittar.