Hey There,
You are right - there is no set or 'right' way to become a builder. I started working as a full time pro-builder in 1989 and it happened as much by luck as by planning.
I could give you pages of advice but instead of wasting your time by making you read advice that would have best given to someone else I'll ask you this - what do you want from building? Do you want to have some fun and make bikes for friend and family and have a cool and fun hobby? Do you want to work for a big company as a builder and earn your living that way? Do you want to set up your own shop and earn a living from building?
These are just a few of the paths you could take and they have require slightly different skills sets and training. They have more in common than not, but they are different.
So what do you think you want to do?
Dave