You don't open a 401k youself, you participate in your employer's plan.
If your annual income is (above $3,000 and) below $100K you can contribute $3K to a Roth IRA (not a traditional IRA) in addition to participating in your employer's 401k plan.
Your Roth IRA can be with a bank (CD usually) or brokerage. A good brokerage choice is
www.vanguard.com and to invest it in their VFINX fund (S&P 500 stock index fund).
You have until April 15 to set up a Roth IRA for 2003's $3K contribution limit.
See the IRS site, tax sites, fool.com, google, etc. for more detailed explanantion.