Practice everyday. Read more books and attend grammar classes. Here are some books and useful links to begin with:
Second, you have to learn the vocabulary. As you know, nowaday people often write short form of words, example : i knw ( i know ), hw r u ( how are you), ect... It's not good. Because if you want to write an application letter so you must use the correct form. :) Using lots of abbreviations will form a bad habit for you.
The last, you try to write as much as posible. By wrting in notebook everyday, how you feel today, what make you happy/sad..ect..
And goodluck :) :)