As soon as I turned 18 I started a mutual fund with $100 down and put in $100 per month for about 8 years. I stopped contributing in 2002 and decided I could do better on my own. While I still own my mutal fund(witch outperforms the market by a little), I consistantly outperform it with my Scottrade account. The more I trade and watch my investments grow the more I learn. I am very confident in my trading now and have even been trading options (161% gain in 3 weeks rules)
Anyhow for you I would also recommend starting with a mutal fund.
senorFrog recommended an index fund, and you can't go wrong with that!
If you don't want to go with a fund, you can open an account at Scottrade for only $500. Each trade is only $7 and each aditional deposit minimum is only $100. Start an account and buy a nice Blue Chip stock (ex: GE, Home Depot, Microsoft, Motorola, J&J, Suntrust) Some of these pay out a nice Dividend too. Then ad to your positions when you have some extra cash.
If you really want to maxamize gains over a long period of time I have 3 words for you "Dollar Cost Averaging".
"Dollar Cost Averaging" means that you buy into your stock or mutal fund on a regular basis. Once a month, once a week.... dosent matter, you will buy on the ups and on the downs but it really helps over time to get the best buying prices instead of buying all at once.
Time is Ticking! Don't wait! Start now, and be patient... so what if what you buy drops 10% if it is a great company just think of it as a good buying oportunity!
Good Luck!