Learning the different disciplines is really dependent on your experience. If you have windsurf or kitesurf experience, wing foiling, will be easier to learn than SUP or prone.
If you don’t have experience with wind sports, the hand held wing will be difficult to understand at first. If you have a great surfing pop up, prone foil surfing would probably be a bit easier.
If you are already great on a SUP in waves, SUP foiling might be the easiest to learn. If you have no experience with board sports, that might be a good thing too, because you are not bringing any habits over from the other sports.
Being on foil is completely different than any other sport.
From what I have seen, most people take 2 sessions to feel comfortable tow foiling behind a boat on flat water. Good surfers only need 1 session. Wing foiling (and already having foiling skills) takes about 10-20 sessions before a rider starts feeling comfortable. Prone and SUP foiling take a longer time to master, in my experience.