Get a Roth IRA, contribute the maximum amount each year. Capital gains will be tax-free and you can withdraw the money later for important things like a house, etc. If you withdraw early you'll have to pay the tax on the gains you would've had to pay if it were a normal account so it's not too big a deal either.
Anyway, to some of you $5000/yr contribution is quite small but it builds up and if you invest well it can turn into quite a nice sum after a few years. Also for you self-employed people get a SEP IRA as well.
As for WHAT to invest in... well that all depends on what others have said earlier. Your risk appetite, time horizon, whether or not you want to generate income off of the investments or just want to build it up for later, etc.