Maybe this will look short post but I like to keep it that way for sure.
So we are starting with one and single question: Ask yourself what niche you are interested in
When you realize what your main interest is and what motivates you to write 20-70 articles and reviews than I am sure it is the right niche. IT can be something that you do every day like playing football, playing with puppies, fashion, sports, obsessions like girls/boys, food, travels places, cars etc (if you want I can write 100 ideas more. )
After this you have to make some research about sub niches you are willing to write about. So if you choose food niche than you have to find what are you interested in like Italian food, Indian food, Chinese food, maybe food recipes, maybe food accessories etc. you get the point right? You are clever enough to get my point only with one reading.
So post for today is finished. Do not leave your work for tomorrow. It is always good to write notes on hard copy like some notebook or open one document and write your ideas there.