Chances are your machine only has space for two sticks. If you bought it brand new it's probably got 2x 1g sticks in it, which means you'll have to buy 2x 2g sticks. Then, changing the RAM isn't usually a big problem. It should be one or two screws to remove the panel then two clips that hold the RAM in.
I actually video'ed the RAM change on my laptop if you want to see it.
Def