Here is a link that'll walk you through the process.
http://www.pcworld.com/howto/article/0,aid,110336,00.asp
This is what I did when i upgraded my ram
1. Find out what model your mother board is. If you still have the computer manual with you then refer to the manual what is the maximum amount of ram supported on the mother board.
2. Rams come in different numbers and specifications like pc100, pc133 etc. The correct ram can be mentioned or found only by the original motherboard manufactrurer.
3. Once you know the number of the RAM, like PC100 . just go to a nearby store, or order online for the amount of ram you need. like 512 MB, 1GB etc.
4. Once you buy it, simply pop open your computer box, and pop-out the previous rams (if any) SLOWLY, and replace with new ones SLOWLY..
5. all set and done right, u'll have your system back.
6. WARNING: Be very careful, if you have no clue what you are doing, then you shouldn't be doing this.
7. If you are a person who wants to learn how this is done, then go ahead and treat yourself..
-peace