Your written language can be improved upon only by practice. You can start by penning down your thoughts or anything else you like to write on a daily basis. As you fall into practice so will your thoughts and eventually they will be well defined and structured. If you are worried about grammatical errors, remember, we all have to start somewhere even if at first we stumble we finally get the hang of things. So start by making mistakes, see where you are frequently going wrong, read more books(that builds your vocabulary) always try to find a synonym to a word you already know. Learn sentence classes( at least the basic) practice and write consistently and you are good to go.