take a quake mod you love and start editing it to a game like the mod you want to build...
im not sure the engines are alike - but the quake source is very very clean. you can start to figure out what entities are and how things mvoe in 3d space. all while getting a lot of bang for your buck seeing your changes come to life from just a few lines of code
you can do the same for an unreal mod of course :) - just find a mod thats open source (like action quake or vanilla quake) and sort of in the direction your game wants to go (fps or top down, realistic weapons/physics or q3 style)