There are a million things to do once you start earning money. You can use your first paycheck to buy gizmos, go out, go on trips, and anything else you can think of, and you should, it's your money! But you should also put some of it away. Even if you just put a small percentage in a savings account each month, it'll add up quickly. The Motley Fool, a pretty good investing education site, suggests investing in the stock market. Not a bad idea either if you know what you are doing. But be careful, and talk to your parents before making any big financial decisions.
