what type of mods is kinda up to you and how much you want to get into on your first time. i would say maps are probably the easiest to get done, but i have never built any vehicle for the game so idk really.
should be plenty of YouTube vids on the "how to" with 3Dmax/Blender (search and watch any vids for these, not just spintires/mud runner stuff).
YouTube and forum threads here and other sites can help with learning the map editor.
i have not used the mud runner map editor yet, but i figure it is not too much different from the spintires editor, so if i am wrong someone please correct me. from my understanding the mud runner editor does have its problems and you might need help with that if you encounter them.
if your PC can play the game then making maps is easy enough, just get the map editor for the game. hardest part of making any map is just getting started imo, figuring out what kind of map you want and where you want to go with it. the rest just kind falls in place really. now if you are wanting to make custom object for your map then you will need something like 3Dmax or Blender for making those custom objects. the rest is pretty much just xml type stuff.