Practice when you can
This tip can never be overemphasized. As an English learner, it is extremely important for you to talk in English at any given opportunity. If you do not have partners to practise with, then try to speak in front of the mirror.
Grammar exercises will help you
Try doing different grammar exercises and find out your weaknesses. These exercises are freely available on the internet. It is only after you are able to correctly assess your weaknesses that you will be able to rectify them.
Find a mentor
Many learners have improved their grammar working with a mentor. A mentor could even be a friend who has a strong command over English grammar. You must speak only in English with your mentor and ask the mentor to point out your mistakes. This real time feedback is very beneficial in improving grammar.