It can be a fairly complex subject, Sunnytwo. Lot's of if's and but's.
Assuming yur phone is capable of doing digital data transmission (some are not and no "wire" is available and no software to make it work), you can call your ISP and it is a just like a voice call. Nothing else involved. But you are limited to in speed to about 14.4 kbps in digital mode. You can also make data calls in analig (AMPS) mode, but you need a celleular modem for that in addition to the "wire".
But the desirable way is to make a data connection through the cell company and use them as your ISP. That typically involves an extra charge but usually opens up the speed to something like telephone land line speeds, 50-60 KB (Kilobytes rather than kilobits).
Cingular's highest speed data transmissions are available only where their latest GPRS service is available. That' smostly in metro arteas right now, but it is expanding.
Verizon has more nationwide high speed data coverage than Cingular at the present time, but since you already have a Cingular phone & plan, you probably want to stick with them.
May I suggest that you check out the Yahoo Group called InternetByCellPhone, which can give you all the info - and help - you may need to learn what the options are and how to buy, set up and run internet connections via cell phone.