You'll certainly want to play the first Portal. Not only does it do a very good job of getting you used to the basic mechanics of the game that'll be needed in Portal 2 but the narrative is kind of integral to the first.

One good thing though is that most digital copies of POrtal 2 came with a free copy of Portal 1, so odds are pretty good that some one can gift one to you for free. I know pathia has a free copy of the PC version she's been looking to give away.