If your going to trade cryptos then I'd suggest looking at the bitcoin category and follow many people there so their posts pop-up on your home page. This way you get a stream of up to date info. Other than that it depends what you like. You earn by posting and engaging in other people posts so pick one to two topics that you will naturally want to post on. I follow "travel" because I fly all over so it's educational for me. If you wanted to post to make money then I'd suggest trying to brand yourself in a sense. If you wanted to do fitness, diet, etc. Then title your posts as if you are a channel if that makes sense. If you randomly post like me then you won't make much, I just have fun. I make my money elsewhere and Steemit is sort of like a game to me.