We've all seen the tight and bright clothing that the pros wear. I'm sorry to report that I've gone minimalist here as well - I just wear my running shorts and shirts and shoes. That's really all you need to start off.
However, if you begin to get serious about cycling, you should consider some good clothing. Good cycling clothing is thin, so you do not get too hot, flexible for comfort, with special material that "Wicks away" sweat (basically, it does not Soak it up and chafe your skin like cotton does). It's also tight, so the wind does not flap your clothing all around and irritate the hell out of you. And the bright colors serve a purpose as well: they make you visible to those crazy drivers!
Cold weather: I do not live in cold weather, but many of you do - in that case, cycling wear thin is good, but layer it on. If you get hot, you can always take off a layer.