One good way to improve one's English is to read a LOT - I mean at LEAST an hour a day ... but make sure that what you read is well written (so real books, not posts on the internet - because internet posts/message boards full of poor grammar!)
When you find a book you like - read it slowly and carefully. Think of how the sentences are written, how they are written/phrased differently than how you would write the same sentence etc.
Also many times copying (hand writing or typing) out sentences from a book can help because when you copy, YOU are still writing the sentence
Also - ask your teacher if they can recommend some good books explaining grammar in "easy to understand English" instead of "technical styles".
Reading children's books can be helpful as well - you can go to the public library and borrow books for free - or read them while in the library.
Of all the things you can do though - the absolutely most important thing is to read read read ... GOOD examples of English - ie well written books.
Two books series that "jump to mind" which are well written, "current" (so you can talk about the books with others) and have plenty of dialogue (examples of people speaking) as well as descriptions (explaining how things look, feel, sound, etc) are:
Harry Potter series (7 books)
One of the many good things with this series is that the first books starts off with a more simple writing style (less complicated words/phrases etc) and gradually gets more complex (bigger words) as the series goes on
Twilight Saga (4 books)
These books have lots of dialogue (examples of speaking well) and use very current grammar styles - which is helpful when working on learning "everyday English". They're fun to read, and are probably appropriate for ages 15 or 16+... if you're younger than that, I'd make sure it's ok with your parents - there is no "sex scenes" etc ... but the last books do talk about "wedding nights", "purity" and other topics that have to do with intimacy.
The more you read, the more you see how things should be said, written ... the more you know when to use tenses (past, present, future etc), and word endings (ing, ed, est).
If you speak (even a bit) one idea that is helpful is reading the words out loud (or silently, but moving your lips) when reading ... this helps connect the written words with spoken words - which helps you connect how you'd say something (aloud when talking) to how you'd write it down.
Also if you speak - then when you write something, try reading it out loud to yourself to see if it sounds/feels/looks "correct" or not.
In time, and with lots of practice you WILL be able to read and write very very well... it's all about be determined, practising a lot, and being patient with yourself.
If you find reading difficult - then start with reading 1 page of a book a day (and understanding what you read), then once that isn't scary/frustrating anymore read 2 pages daily, then three pages, then four....
Soon you'll be WANTING to read more, wanting to finish a chapter etc ... honest
It took me a very very long time to learn how to read (and write) and it was VERY frustrating at first - but I was very fortunate to have a teacher that showed me that reading is something you can learn to love ... and reading can take you ANYWHERE in your imagination. She taught me to LOVE reading - and now I have thousands and thousands of books and read books 1-2hours everyday at LEAST, plus reading on the internet etc
You can do it.