Hello,

The basic step towards learning or writing a language is to have a strong hold on its grammar, the best book regarding this is wren and martin book, you can refer Norman Lewis little book named Word Power Made Easy for vocabulary building, you can read The Hindu on a regular basis, it's editorial are written by prolific writers, you can read novels of your favorite genre, watch good movies or TV Shows, you can also follow blogs like Harvard Business Review, this will give you opportunity to learn and understand the vocabulary and writing style of great writers, you can start blogging or creating a personal website, you can start with writing down about things in general or your thoughts, if you've inclination towards creativity you can start writing verses initially and then move to short stories and later on longer format. There are various channels in YouTube and even in Internet to provide such lessons, you can avail that.

You can also join MOOC courses such as EdX, Coursera, Future Learn, Alison Courses, Swayam etc