"What if you know how to write an essay but have no idea what to write in it "
Common issue with everyone. What I suggest you, read as much as you can, novels, history books, anything you like (Of good writers), in any language Urdu or English(I prefer Urdu books). It will improve your expression and sentence structure. Both of them are key players in any writing. Many people tell you write as much as you can, but what I believe, writing is useless, unless you don't know what to come first and what should be at the end of your article. So conclusion is, first go to any library, issue few books you like, and then just read them, there is no need to remember anything, just enjoy the reading with music if you like. After 1 month, you will feel huge difference.