It's never too early to begin planning for your retirement. If your employer offers a 401(k) or something similar start investing in it. If your employer doesn't offer a retirement plan, then one of the best ways to prepare for your retirment is to set up an Individual Retirement Account (IRA).
The purpose of the IRA is to serve as a personal retirement savings plan. Anyone who works can contribute to an IRA, with the earnings on these investments tax-deferred until the date of distribution (read: your retirement). Additionally, many individuals are permitted to deduct at least a portion of their contributions to the IRA off of their taxes. I recommend most individuals set up Roth IRAs if you can, where your contributions are not deductible off your taxes, but your withdrawals at retirement won't be taxed either.
It doesn't take long to set up an IRA. Most online brokerages can do so in about 15 minutes.
You must begin taking distributions from your IRA no later than April 1st of the year following the year in which you reach age 70 1/2.
This is just quick and general overview of IRAs. The rules are different for Roth IRAs and traditional IRAs. Before setting up any type of IRA, take the time to talk to a financial advisor or tax accountant, making sure that you have a grasp on your personal options. You want to make sure that whatever option you choose is going to be the best for your needs.
You can learn more about IRAs online from the Internal Revenue Service here: http://www.irs.gov/taxtopics/tc451.html