Read as much hub information you can get your hands on regarding hub facts and hubs about making money, generating traffic and setting up analytic trackers. Then, write about what interests you.
I have written about personal experience; news and current events; my favorite electronics - whatever pops into my head. For me, to keep writing is by far the most value I can get from having a hub. The more you write - the better you get. The rest will come in time.
Good luck and welcome to the community!