I would start by talking to your current or previous employer (the one who sponsored the 401(k) plan). I'd contact HR. Another option might be the Summary Plan Document (if you have it) which should explain how to make withdrawals or whether they are allowed if you are still working. Also there most likely is some sort of Third Party Administrator, often payroll companies (like ADP or Paychex), that can help.
If you are less than 59 1/2 you will pay taxes and penalties (other than in very specific situations) if you make a withdrawal.
Bottom Line: Start with your employer and ask for the Summary Plan Document.
Hope this helps.
Everything Curt mentioned is great stuff. I will add that if you are indeed younger than 59 1/2, the best thing would be to try to avoid taking from your 401k unless absolutely neccessary.
If by chance you have moved on from a prior employer and you are looking for a way to move your money away from that employer, there are better options that just cashing out your 401k. You could roll over your 401k into an IRA. I would suggest finding a local fee-only advisor who can give you advice on what to do next.
Daniel, every plan has a plan administrator. Contact HR and get the number of the plan administrator. They will provide the forms to pull out your money.
With that said, a retirement plan is not just a savings account. It is designed to encourage you to save for retirement, and in return you are given certain tax advantages. The money is tax deferred, meaning income tax has not yet been paid, and if you withdraw it, you have to pay income tax. Plus, if you are not age 59 ½, or 55 if you left company, there is another 10% penalty. So, you can take your money out, but a big chunk is going to go directly to Uncle Sam. I realize I might be preaching to the choir, but my suggestion is that you may need it more when you do reach retirement than you actually do now