First of all, if you don't know that much about computers, overclocking may not be the best thing to do. Anyway ...
Start with downloading the Voodoo 3 overclocker. It's pretty easy to find, I think the 3dfiles site has it.
Launch the downloaded file, choose Voodoo 3 3000 in the pop-up menu.
The prog will now appear in your display properties menu.
Check the "allow overclocking" box, then set the value you want.
BUT, it is very important to be humble, i.e. do it by steps fo not more than 5 MHz each time. Restart your comp, and launch a 3D intensive application for a while, to check the stability of your card. Quake 3 is a good choice.
You can increase the speed, following the same steps, until when playing you start to have artefacts on the screen. There you have reached the upper limit.
This is for standard cooling. You can then play around with 486 or Pentium fans to put on your vid chip (and memory if you can).
That's pretty much it.
You could ask some specialists as well. Look for threads about overclocking.
Main rig: Athlon64 x2 4200+ | Asrock 939N68PV-GLAN | 4x512MB PC3200 | 120GB Seagate | Samsung DVDRW SH-S203D | VX550W PSU | WinXP Home SP3 / Ubuntu 7.10
Headache generator: K6-2 500 | FIC VA503+ | 2x64MB PC133 | Voodoo 3 2000 PCI | SB32 ISA PnP | 15,2MB IBM | Xubuntu 8.04 / DeLiLinux 0.7.2
Lappy: Compaq EVO N115 | Mobile Duron 1GHz | 384MB PC133 | VIA KM133 | Xubuntu 8.04
A big thanks to MS's Vista for making me look into Linux distros!