This is better posted in the Cingular forums, since you will need some Cingular specifics, but basically assuming you are using winblows, you setup a new dial-up networking profile, tell it to use your phone as the modem (either over bluetooth or cable), and use *99****1# for the dial-up number (I think that's correct for Cingular, it's just *99# for T-Mobile). This will use your data plan, so if you aren't using unlimited data, you are going to get an assload of charges... plus I've heard that Cingular frowns on tethering, so even with unlimited data they might charge you, I don't know.
-olly