Re: first project
Welcome to the forum.
I would not attempt a 20inch wheeled bike.
A cruiser (24) is probably too small without getting the torch out.
An older steel mountain bike with horizontal dropouts and BMX components (stem, bars post, etc) would be unique and simple to build.
My motorbike is an older single speed steel mountain bike, but I have to use a chain tensioner because it has vertical dropouts.
“You're only young once, but you can be immature as long as you like" -Unknown