I haven't really sketched for years, but used to a lot when I was in my teens and then my twenties.
My Art teacher always marked me with an A so I couldn't have been that bad, and said that I had a flair for drawing cartoons.
He said he knew someone in London who was always taking trainees on and putting them through an apprenticeship of sorts, but I never followed it through.
The first sketching obsession I had was with sailing ships, like this one below (not mine, but roughly the same quality, although I always had trouble with the sea).
The others were Superman, Batman, Popeye etc., until I discovered girls and also Frank Frazetta at about the same time.
Now, if I could sketch and paint like Frank, I would be very happy.