It is easy to do yourself. Just get a large soft towel. Shut down the computer, unplug everything and place it on the towel - you want to access the bottom of the base. With a regular (phillips, I think) screwdriver, take off the bottom panel and there you are.
I did it myself when I had my iMac 15" (I now have the 17" version which already has 512MB RAM installed). Be prepared for a VERY tight fit for the chip and a very specific way to install it. The chip slides in but you have to apply just the right amount of force to get it to snap in fully; just be careful you don't snap the RAM while you are doing it! If you feel safer, any Mac store can install it for you for a nominal fee, though the instructions that were just posted should be sufficient.
The maximum you can have on that cute baby is 1GB. There is one module that is factory installed. That is probably your 256 MB that shows up. So you can buy the largest chip you can find - PC133 - 144 pin SO- DIMM