The first and foremost thing you should begin with is, reading. Read newspapers, editorials, and magazines. Also, make it a habit to talk to your family and friends in English. Apart from this, it is essential for you to go through the basics of grammar.
Practicing frequently asked question types such as parajumbles, para completion, summary, FIB, and critical reasoning can help you improve rapidly. It is more than essential to go through every mock that you've given. Mark out the difficult words from the RC, and learn their meaning & usage in the context.