You don't need to be an expert, you just need to know more than your audience. What I mean is, if you tailor your blog towards novices then no one will attack you if you don't know everything or even if you've gotten things wrong. However if you tailor your blog towards veterans in the field then you'd better know your stuff. The beauty is that as you learn you can teach more advanced subjects always catering to a group that is where you were a few months ago.