Teach yourself! 
My suggestion is to start off with simple stuff, i.e. adding an item. You can create model files with Qubicle or you can use StoneVox (free, found on this forum)
Adding them is straightforward. This thread got me started on modding:
Ideally, someone should make a dedicated post with the basics of adding a new object to the game!!
Other tips:
When modelling, take a pre-existing model similar to what you’d like to add and change it up.
Start small - maybe add something simple like a bucket or a fencepost at first, then work your way up from there.
Haven’t had a look at @voxel_pirate’s vids, but I hear they’re great… Maybe some weekend youtubing for myself if I have spare time 