I went to the local Flea Market and one of the inside booths was a "bike Trader".... you can buy used bikes from this guy... I saw an awesome Schwinn "Del-Mar" "beach cruiser" bike for my 14 y/o son. The guy wanted $100 for the bike. It was in excellent condition. It was used by apparently not very much. I traded my 10 year old 10speed bike plus a 5 year old 10speed girls bike, plus gave the guy $50 and the Beach Cruiser bike was under the christmas tree for my son. Excellent deal on an excellent bike.
If you don't want a used bike. . . go to Target or Walmart (despite some of the responses here). You will get a good bike that will suffice. As a youngster, my parents always bought me the cheapest bikes, and while my friends always had 10 speed racers, my parents bought me a 5speed... however, I was NEVER beat in a bike race and I was very proud of that fact. It proved a point in my mind that it is the RIDER that makes the bike, not the bike that makes the rider.