Like OwlGo said, a good point to start is by having an idea! Try to keep it simple at first - so you can learn - so stay away from professions or biomes and such for now… Think simple to start off: what new thing would you like to see in the game?
A new tree? A new piece of armor or weapon? New furniture?
Then you can get started from there!
I feel like this recent thread has a lot of good advice, especially the modding guide linked by BrunoSupremo, so take a look at it
