If you find you enjoy it and will be riding any distance or frequency, make sure the bike "fits". A seat a little too high or low can cause a lot of grief. I bike a fair amount- it's good exercise and with gas the way it is, I'm thinking of riding to the store more often (even though it's only a 3 mile round trip).

As said before- always, always wear a helmet. You might look a little cooler without one, but you'll look really uncool drooling from a serious head injury. And they do happen.