What kind of bicycle should I get?

I am interested in getting a new/used/recycled bicycle for myself. I have about $100 to spend, would like something that changes gears, but still looks classic, and has the ability to add attachments like a rear or front basket. I plan to use it to ride around my neighborhood, and possibly back and forth from the farmer's market in the center of town. I have never bought a bike for myself, so choosing one is proving to be a little difficult. Where should I look? What brands are good? Should I go to a bike shop or to a big box store?