Today, I went to athe movie theater alone.

Better word choice, sounds more natural.

This is whyat is because my familly is really busy.

This is why is usually for actions caused by oneself.

I watched “F1,” I don’t wanna spoil anything, but I have a one thing to tell everyone.

These days, I’m really into action movies.

After I watched this movie, I went to a Goncha.

I druank a grape juice.

I ordered or had grape juice sounds more natural but that’s ok.

It is a seasonal limitedlimited edition drink.

Today, I went to a theater alone because my family is really busy.

This is why my familly is really busy.

You can say "because"! "This is why" sounds a little awkward!

I watched “F1.” I don’t wanna spoil anything, but I have athere's one thing I have to tell everyone.

These days, I’m really into action movies.

After I watched this movie, I went to a Gongcha.

The tea place right? ^^

I druank a grape juice.

This is whybecause my familly is really busy.

I watched “F1” - I don’t wanna spoil anything, but I have (a single / one) thing to tell everyone.

"one" and "a" don't combine like this, you can use "a single thing" or "one thing"

These days, I’m really into action movies.

Since you're into more than one action movie, it's better to say "action movies" here.

After I watched this movie, I went to a Goncha.

I druank a grape juice.

It is a limited seasonal limited drink.

This is whybecause my familly is reallywas too busy.

You could combine these: Today I went to a theater alone, because my family was too busy."

I watched “F1.” I don’t wannat to spoil anything, but I have a one thing to tell everyone.

Wanna is very slang; it should be avoided for English practice

These days, I’m really into action movies.

After I watched this movie, I went to a Goncha.

Just a comment, but I don't know what Goncha is. Google suggests it's bubble tea - is that right?

I drunkhad a grape juice.

For eating or drinking something, we tend to say "I had X." But if you specifically want to use the verb to drink, "I drank a grape juice" would be correct. Drunk is used as an adjective when someone has had too much alcohol.