Here's a quick and dirty way to plan for retirement....
At the beginning of the year, figure out how much money you want to put away for retirement, say $9,000 a year. Look into your companies 401k plan, and if they have a match, take advantage of it, and max out your "free money" from them. For example, if you company matches the first 5% you put into the 401k at 100%, then that is 100% risk free gains. So, ideally what you would want to do it max out your free money from your employer, for our example, we'll say your employer matches the first 3% of what you put into your 401k. Now lets assume your salary is $50,000/year, so then you ll put $1500 away into your 401k plan through your employer, as you put that away, they hand you (into the 401k plan) another $1500...you can not beat that risk free return in the open market!
So now (not including the match) you are $7500 away from your goal of putting $9000 away for retirement in the current year. 401k's can be limited to that amount of funds the offer and the investment you can invest in through them, but by putting money into them you get the match and you reduce your tax liability by the amount you put away. 401k's are also taxable when you take the money out down the line, SO, next you would want to max out your Roth IRA.
A Roth IRA is a retirement account that the money can grow tax free, and you can take out your contributions anytime you want without any tax implications. You can invest in any type of security in a Roth IRA (stocks, bonds, mutual funds, etc...) so that is a huge advantage from the limited funds you'd be able to invest in through the 401k. The limit this year if $5000 for those accounts, so you will want to put $5000 into the Roth IRA throughout the year...that leaves you with $2500 to still put away during the year to reach your goal.
Go back to your 401k and up your contribution's there to cover the other $2500 that you need, and that way you have put away your goal of $9000 towards retirement and it is all growing tax free! The way I described takes advantage of the benefits your company offers, and the benefits that the government is giving you with the Roth IRA's.
Any questions on anything, let me know and I can clarify anything.