By going out to Best Buy, purchasing a RAM chip, opening up your computer, and sticking it into the motherboard.
If you're not experienced in this kind of thing, the Geek Squad is your friend. Bring your computer in and say "I need 512 MB of RAM". Otherwise, you'll need to know what kind of RAM is compatible with your motherboard, what slots you have open (type, how many) and max speed of the RAM. And whatever you do, don't touch anything inside your computer without being grounded.