Need to be skilled in the following disciplines: 3d modelling, texturing, JBeaming. Not one of which can be summarised into a simple tutorial.
The most easiest mods to make are skins or terrains there are a lot of tutorials on making a map plus they are a really useful mod as people always want new terrains to drive on and you only need to use the terrain editor that is shipped with the game (you can use external map making tools like world machine only if you want to make a complex map with mountainous terrain)