I'ved never heard of it though, until my brother told me about it a few weeks ago.

'I've...' means you've never heard of it until this current moment (implying that you still haven't heard of it)
'I'd...' means that you never heard of it until a moment in the past, when your brother told you about it.

So, I looked up a few of the bands playing, tookgrabbed my camping hammock and went thheaded overe.

This sentence isn't wrong, but it isn't natural. All three of my changes are kind of difficult to explain - I mostly just went off of the magic English thing in my head.
The comma shouldn't be there because there isn't a pause between so and I.
Grabbed sounds more natural in this context, but it isn't absolutely necessary. "Took" I think is used when you're going to do something with the thing you took, but you didn't in this case. Grabbed would be used in this context in which you wanted to take something along with you.
I can't think of a good explanation for why headed over sounds better. Sorry.

The reason, why I've never been to a festival, is the price.

These events ay're quite expensive.

Also, thanks to my brother, I couldwas able to work part time in the kitchen.

"Could" means that you had an option. "was able" means that you had an option but also took advantage of that option

In return I got free entryin for free and some festival-money for beer and food.

or "was granted free entry", but that sounds a little too sophisticated for this journal

On the first day, me and a friend and I sold food to the - often drunk - coustuomers.

This is a mistake that ALL native speakers make and we're told in school to fix. You can ignore it if you intentionally want to make "native-like" errors.
Also look up the difference between a customer and a costumer :)

While working, we couldenjoyed / were able to enjoy the concerts and each others company.

At the beginning of my shift I was tired already, but alsoI was already tired before the shift started, but nevertheless motivated to learn how to make proper, round crepes.¶

I've been trying to think of how to explain why this is better but my brain hurts.
Feel free to ask a question and I'll make my brain hurt some more.

Little did I know that for the next four hours not only wouldn't I I not be able to take a break, but also hadwould have to deal with demanding costumers, missing incredience??? and a lack of support from the festival producers.

At some point, the circle-movement of making a crepe wasbecame hipnotising for my tired brain.

At some point implies a change that happened, thus "became"

I remember that at one timepoint, I starred at the baking dough for way too long, forgetting the world around me.

One point describes a small time slice. One time describes more of once incident among many.

When I stenapped out of my trance, the costumer next in line looked at me worried... and ordered a ham and cheese crepe.

"snap out of a trance" is the right expression here.

I didn't listen to a lot of concerts that second day and learned that a free entry isn't worth working at the crepe station.

But let's look onat the bright side of it: I defeinitely knowlearned how to make proper, round crepes.

NeverthelessStill, I don't know how tasty they are.

Nevertheless is kind of a fancy word in English and shouldn't be used in this context.

My fFirst Festival

I'ved never heard of it, though, until my brother told me about it a few weeks ago.

So, I looked up a few of the bands playing, took my camping hammock, and wentheaded over there.

The reason, why I've never been to a festival, is the price.

But this festival was really convenientreasonably priced for me.

We're generally taught to never start sentences with "but," but native speakers do it anyway. Just something to point out!

I also replaced "convenient" with "reasonably priced" to be more precise.

Also, thanks to my brother, I could work part time in the kitchen.

In return, I got free entryadmission and some festival- money for beer and food.

On the first day, me and a friend sold food to the - often drunk - cocustuomers.

While working, we could enjoy the concerts and each other's company.

On the second day, however, I worked at the crepe station.

And oooooh boi that was a nightmare.

Haha, if this was a formal composition I would remove the oooooh boi. Otherwise, I am so sorry that you experienced this.

At the beginning of my shift, I was tired already, but alsoalready tired but still motivated to learn how to make proper, round crepes.

Little did I know that for the next four hours not only wouldn't II not be able to take a break, butI also hadwould have to deal with demanding costumers, missing incredience and a lack of support from the festival producers.

Not sure what you meant by "incredience" here.

At some point, the circle- movement of making a crepe was hiypnotiszing forto my tired brain.

I remember that one time, I starrted at the baking the dough for way too long, forgetting the world around me.

When I stenapped out of my trance, the coustuomer next in line looked at me worried... and ordered a ham and cheese crepe.

"..." isn't really necessary.

I didn't listen to a lot of concerts that second day and learned that a free entry isn't worth working at the crepe station.

Nevertheless, I don't know how tasty they are.

Alt: That said, I don't know...

My fFirst Festival

It's a rather small festival that takes place every year, just a few kilometers from where I live.

I've had never heard of it though, until my brother told me about it a few weeks ago.

I'd is also fine here if you want to be more casual

So, I looked up a few of the bands playing, took my camping hammock and went there.

A bit more natural sounding - though you were correct!

The reason, why I've never been to a festival, is the price.

But this festival was really convenient for me.

"For me" is a little redundant here - though technically correct!

Also thanks to my brother, I could work part time in the kitchen at the festival.

Just to add a bit more context

In return I got free entry and some festival- money for beer and food.

On the first day, me and a friend and I sold food to the -, often drunk -, coustuomers.

On the second day, however, I worked at the crepe station.

And oooooh boi that was a nightmare.

Leaving the "oh boi" slang in as it seems to be intentional!

AI was already tired at the beginning of my shift I was tired already, but also motivated to learn how to make proper, round crepes.

Just for a more natural sounding flow

Little did I know that for the next four hours, I would not only wouldn't I be unable to take a break, but that I would also hadve to deal with demanding costumers, missing incgrediencets and a lack of support from the festival producers.

At some point, the circle-movement of making a crepe was hiypnotising for my tired brain.

"Hypnotic" would also work here

I remember that one time, I starred at the baking dough for way too long, forgetting the world around me.

I might have to start making crepes - this sounds kinda fun!

But let's look on the bright side of it: I defeinitely know how to make proper, round crepes.