You really have to be careful about investing small amounts of money because sometimes the fees could drastically cut into your returns. It's a good idea to get investing, but you might want to just wait until you can meet the miniums.
That said, if you still insist, this is a good resource:
More IRA Options For Those Starting Out » Blog Archive » My Money Blog - My Path to Financial Freedom
You can also open an account at
Sharebuilder or
FolioFN (I prefer FolioFN) and purchase exchange traded funds such as
SPY.