Ask as many questions as you want - if they seem unwilling to give you an answer, or kind of laugh in your face, I suggest moving on. Try to test ride a couple of different bikes to see what is right for you.

I was completely clueless when I got my first bike. I just knew that I had to have one. I took it out for a 10 second test ride (I was really wobbly on the bike as well). Luckily for me, it turned out the bike was a good fit, and I had BF with me, he's been cycling for years, so that was helpful. If you know somebody who rides, I suggest you take that person along.

Good luck!