Solid advice by Hitman so far. There are two considerations to be made here. First, what type of retirement account. Second, what to put into it.
There are a few basic types of retirement accounts. 401k's and IRA's. 401k's are administered by the company you work for. IRA's (Individual Retirement Accounts) are opened by yourself through a company like Fidelity. Both contribute pre-tax money. That is important to know. And what it means, is that you put your money into the account BEFORE paying tax on it. So if you are taking home $1000 per check after taxes now, and you decide to start putting in $100 to a 401k or IRA, you will still take home about $910 -- see, you'll save a little bit because you're not paying taxes on that. Anything you put into the accounts will grow and grow until you retire (at 60 years old), then you have to pay taxes on it when you take it out.
The second type of account is a Roth 401k or Roth IRA. In those, you put money in AFTER tax. So if you take home $1000 now, you can put $100 in there and take home $900. The good thing is, once you get that money into the Roth account, it's out of Uncle Sam's hands forever.
The choice to be made between the two (Roth or Regular) is a tough one. Basically, it depends on if you think you'll be in a higher tax bracket now or when you retire. That's a question you'll have to answer.
Ok, so that's the first decision. Now that you have the account, it's time to decide what to put into it. Hitman is right that an S&P 500 index fund is a great choice. There are also funds built specifically for retirement accounts, they will ususally be called something like "Retirement Fund 2050", and the year is when you expect to retire. It will be managed more aggressively when you're young, and more conservatively when you get close to taking the money out.
If your company is big enough to offer retirement accounts, ask whomever is in charge of it, they will help much more than we can. But you're definitely doing the right thing getting started NOW. It's a game of compounding interest, and the sooner you start rolling that snowball down the hill, the bigger the ball will be once it gets to the bottom.