First, you need to determine what kind of ram you have, so you know what kind to get.
Run this program, called cpu-z. When you run it, click on the memory tab, and it will tell you what kind of memory you are running. You want to buy something similar to that.
www.cpuid.org/download/cpu-z-130.zipFor example, if you are running PC2100 DDR, then you want to buy at least the same. You can buy PC3200 or something much higher if you want and if the price is right, it will just underclock itself to PC2100 levels.
You could also have SDR ram, or SD-RAM. That is not compatible with DDR of any kind.
Anyways, when you know what type of ram you have, go on over to
Newegg or some other store (newegg is as good as it gets, they are the best) and order you some more ram.
You also might want to see how many RAM slots you have open in your computer also, before ordering.
Its not that hard to put new ram in, there are plenty of guides with pics floating around the net.