If you want to add more memory, download and install a program called Everest which will tel you the model number (and brand name) of your motherboard, and also which type of memory it takes and also the maximum that your motherboard will accept.
Armed with that, you can buy more (or even go to crucial.com and enter your motherboard details there for confirmation of which type of RAM your motherboard will accept). I'd aim to add at least an additional 512MB but the more the merrier.
From there it's just a matter of buying it and adding it in. RAM just slots into your PC - it's dead easy to "install". There'll probably be websites with pictures showing just how simple it is.