Alberto: Science's teacher.

I want to learn English becuase my goal is to teach Science subject in English in my scoll school for the next year.

Alberto: Science's teacher.

I want to learn English becuase my goal is to teaching Science subject in English in my scoll school for the next year.

Or you can replace your sentences,

I want to learn English, because next year I am going to teach the science in English. 

Try and try to read many sentences in English. Don't forget to learn more the gammar and spelling too. Your writing will be better.

Good Luck!