It's easy, start a mod thread and ask for help. Modding is only slightly harder than posting.
Not really, scan this page for a sticky called "The Ultimate Introduction to Modding | Starting out? Read this!". If you can find it, you are off to a good start!
Do the tutorials even if they're outdated by several years. You'll need to learn the M&B Python language, 3d modelling, texturing, animating, etc. But while the visual stuff is nice and exciting, the coding is the thing that makes your mod tick.
SO download the Module System, do the tutorials, etc. At least ad one new faction with its own town(s), castle(s) and villages, plus its own king, knights, troops tree etc... that works and then go for more complicated stuff