Complicated day !!

Now I study in high school. Education in Thailand has two programs to study. I have to study in science program because my parents want me to study that, but I love languages. I want to study in arts program but they dont want me to study that. Today it is so bad because I have to choose to study in special science. That's so hard and so expensive, but I have to study that so I can gain entrance to university....
Now I think my life will be complicated...
I won't have much time to play and watch tv anymore...

 "...I have to choose to study science" or "I have to choose to specialise in science" or "I have to choose to study specifically in science".

"...uses a lot of money" is the correction for "have to use money so much". It's better to just say "expensive".

You've used 'that' so many times for so many different things it's confusing what you're talking about.

"...dont want me to study that." "...but I have to study that..." - These are the corrections, but you don't need to keep repeating yourself.

Complicate day !!

Now im study height school .Education of Thailand has two program to study .I have to study in science program because my parent want me to study that but i love language i have to study in arts program but they dont want to study that.Today it is so baddd. Because i have to choose to study in special science .That so hard and have to use money so much!! But i have to study that because i can entrance that ....
now i think my life will be complicate...
i wont have much time to play and watch tv anymore...

Complicated day!!

I'm studying at high school now. Thailand has two programs of study and I have to take the science program because that's what my parents want me to study. I want to take the arts program because I love langages but my parents don't want me to study that. Today is a really bad day because I to choose to specialise in science. It's so hard and very expensive, but I have to study it so I can get into (university/higher education) [not sure which one is more applicable to your situation!]

Now I think my life will be complicated... I won't have much to play or watch tv anymore...

Sorry to hear about that :( I'm sure it'll be easier than you think it will be, it always is!!