Mostly it depends on where you want to specify, as i never saw a dev that mastered and loved all kinds of work in PR

Wanna code? Just change random values in .con and .tweak files in mods/pr/content zips, see how they affect game, change more, get more knowledge. Or dive into python server code in mods/pr/python/game/__init__.py
Want to be a texture dude? Download BFmeshviewer, open some bundle\static\skinned mesh file, see how it looks and which textures it uses, change them in your favorite photoshop\gimp\MS paint version, see how your new mlg soldiers look.
Wanna be a mapper? Download bfeditor, set up your dev version of mod, unpack some other map, see how things placed, create your pet project map from tutorials around, start creating new maps once feeling skilled enough(the truth it's never through

).
Modelling\animating is your choice? Make a robot bender in your favorite modelling program(max\maya\blender\zbrush), export it to max9, export to game. Maybe after all that you'll be able to animate it
