First of all, learn to spell the words correctly. Make sure that you understand the words that you are going tu use. At least write simple sentences if you can't write long sentences. Don't use hard words and long sentences just to impress people, it'll be useless if the vocabulary/ grammar is wrong. Other than that, read lots of english books or watch english movies. Everytime there's a word that you don't understand, translate it. Hope this helps!