how? what software tools?
Does it require coding?
How do I create graphics?

Modding isn't that hard really, just look at released mods to see how they work. Mainly you're gonna have to deal with xml files and the RWR voxel editor. If you want to make vehicles, you're gonna have to get a program to create 3d models and export them to .mesh files. For maps, it's a complicated process and devs haven't yet released some kind of tutorial on how to make maps.