Like Adv. Girl said, it really does depend on what kind if riding you plan to do. But also remember, you're entitiled to change your mind and style. So if you choose a road bike, and find out it's not your thing, don't make yourself miserable, but find something else to ride. If you like the wild side, try a mountian bike. There are lots of old railroad beds, canal towpaths, and unpaved roads where I live, so when I got back into biking, I chose a hybrid. After less than a year, I realized that I liked going fast, so I got a road bike. I ride that most of the time, but still ride the other once or twice a month. It's great for long slow rides, when I go out with friends to socialize (ride, eat,ride, drink, ride, eat, ride, drink, etc.)
As for clothing, bike shorts do make a difference. You don't have to get everything at once, but if you do a lot of riding, you'll appreciate having the right gear.