I would encourage you to read the success stories, so that you can get an idea of exactly what's possible here. Also, see what the other ordinary mortals are earning - for instance, I have a couple of hubs comparing my HP earnings to my income form other sites.
Finally, don't make writing for money your sole goal. Make some of the things that you write about simply things that you want to write about. Oddly enough, I've experienced the most success with those things I wrote about just because I liked them, not because I thought there was a pot of gold at the end of the rainbow. (Not that there's anything worng with writing for money; I'm just saying be sure to mix in some fun stuff, too.)