Retirement seems awfully distant for people graduating from college right now, but if you want your money eventually to work for you, rather than the other way around, the magic of compounding -- especially tax-free and employer-matched compounding -- translates into "the sooner, the better." The Wall Street Journal had a great article on that subject over the weekend. Usually online WSJ content requires a subscription, but Yahoo Finance reprinted the article here. Once you've maxed out your employer's 401(k), you'll want to be shopping around for things like Roth IRAs. I like Fidelity's ease of use and low-cost funds -- check them out here.
By the way, I also recommend an online WSJ subscription to any college student or recent graduate (also a hint to people casting about for graduation gifts). It costs about $10 a month or $100 a year (if paid up front), and current students should find out whether they have free access through their schools. Despite the name, the WSJ actually covers much, much more than finance, and I can't think of a better daily newspaper to read to prepare for the real world.