First of all, the Writing test is different for Academic and General training Module of IELTS. I am going to talk here about the General training Writing test.
It actually has 2 sub-tasks. The first one is to write a letter according to a scenario you will receive, using not less than 150 words. The second task is to write an Essay on a given topic, present and justify an opinion or give a solution to a problem, using not less than 250 words.
Nothing to worry about here! Once you’ll start using a certain structure which I’ll explain later on for the letter and the essay in addition to your imagination, it is a piece of cake. This task requires a bit of training, but after you write a few essays and letters you will be well-prepared for it and you will feel confident.