If it's artwork you're interested in, you might take a look at the
texture replacement thread. We've got lots of unlicensed textures we need to make GPL and this involves Photoshop work. You'll need to learn a bit how 2d textures are used to create 3d environments, though. The
Artwork page says a bit about this from a more technical angle.
And in terms of learning, you might want to take a shot at
mapping. It's definitely something learnable and will give you a sense of how all the different components of the game come together in the battlescape.