The easy answer for someone your age is the Vanguard Total Stock Market Index Fund (VTSMX). But it's really important that you understand why you're doing something, rather than just taking advice from random strangers on a personal finance forum.
The first sentence is good advice. The second is the is the best advice you will ever hear about anything in your life. There is enormous value in understanding what you do, thinking for yourself, and making decisions on your own. No one knows what matters to you better than you do.
I'll echo flinch13's advice about putting your money into a Roth IRA. But I'll hold back on agreeing that a target retirement fund or automatic investments is the best thing to do. I happen to like thinking about where my money goes. If you want to just "set it and forget it," then flinch13's advice is sound. I prefer to understand what I'm doing and take a more active role in my investing.
Just to add the voice of another stranger...I like the Roth IRA as long as you are eligible. I also suggest the Total Stock Market fund for the reason that Vintek says - you will be thinking about what you are doing rather than just being passive. But the reality is that the target retirement fund is going to be almost 100% in stocks for a person your age so it really hardly matters until you get into middle age.
But if you just want to narrow down the choices so that you can research them, I suggest sticking with the funds with "index" in their name as well as the target retirement fund of the appropriate year. That will give you a list of a dozen of so funds that should be educational to research.