If you're creating maps, all you really need to know is level design, which there are a billion UEd tutorials on the Web (I recommend Architectonic at PlanetUnreal), and the fantastic video tuts available on 3dBuzz.
Knowing modelling and programming and stuff like that will be important if you are designing a lot of custom content, but UEd is such that you can still create a halfway decent map just using the legos that came with the retail game.