Test ride, test ride, test ride! Someplace like REI or a good sporting goods store will have lots to choose from, and any decent LBS will have a variety of types of bikes you can take out for a spin.

Save trying the road bikes until you've test ridden a few hybrids and mountain bikes, though, if you're just getting into things. First time on an unfamiliar road bike with those mustache handlebars can be a bit intimidating!