My vocabulary is not good..so what should I do to improve it ????

Read english newspaper daily and keep a dictionary app with you. Whenever getting any word out of your knowledge, search immediately on dictionary and then proceed. For better and fast result, try to go through dictionary daily mugging up some 10-20 words while revising them regularly and if possible then also use them in random journals and paragraphs or in diary entry. Trust me if you follow these things consistently then you will observe a drastic improvement in your vocabulary yourself.