Things you know, love, like, and are passionate about. But mainly what you know. Politics? Food? Books? Social issues? Music? Science and religion, as have already been mentioned? Family? Hobbies? Pop culture? You have a voice, and you can use it as you like, so long as you are responsible. Things you'd like help with, things you worry about.. People like knowing they aren't alone. What about the subject of your education? Or your country and it's culture? There are a lot of people who would be interested in that. Or your life and aspirations and ideas. Or a potluck of everything you like, with direction and appropriate planning. Like lacrecia said, it has to be something you're interested in and that makes you think otherwise there won't be much sincerity in it and you'll stop wanting to continue after a while.