How should I present myself at a job interview?

I am in my mid-30s, and am quite intelligent, but don’t know how to show that I am w/o appearing pretentious or arrogant. Plus, I am not good at speaking (I’m better at writing), so people tend to dismiss me as incompetent or “not that bright”. I bombed a job interview yesterday. I didn’t know how to fend off my nervousness. It was a harrowing experience. Of course, I didn’t get the job. Any tips, please?
Most of my experience is in self-employment and volunteering (self-directed initiatives). I have a big problem w/ expressing myself orally. It’s just the way I am, and unfortunately don’t have the time nor money to take classes. I’ve got a job interview on Friday and another the week after.