Seems like you know to max your 401k. Even if you don't at least max employer matching, as already said. It's free money!
However, look on your next 401K statement, or ask your HR to see the various options. You don't need to know much, but look for something that says expense ratio. (that's the fees) In the reading already given, you'll learn that that is really what keeps your returns down: a high expense ratio. So since quite often the options in employer packages are not the best, I'd look to see what the lowest expenses are, and see if there are any index funds. Out of those, pick the one with the widest exposure. Such as a total stock market, or a s&p 500 index fund. If no index funds, choose the lowest exp ratio with the widest exposure. Or write down all of the options, and post them here, and have everyone weigh in on the options. while making the final choice will be a personal choice, I know we'll be able to rule out bad options, and tell you why. the why will be the most valuable information. Also, a target date fund (sometimes called lifecycle funds) may be a good "set and forget" option, if your plan has it, and you are overwhelmed by all the options. Just make sure it's a bit lower fees.
PBS.org: the retirement gamble. In addition to the reading, this will explain investing in a way that one, will make you afraid for 90% of people, and two, will bring all of your reading together, and teach you about index funds.