Pre-T, bodybuilding isn't going to change your shape very much, except by toning muscles and helping get rid of fat. What it WILL do is increase your strength and, done correctly, your flexibility; at this stage, I'd focus more on strength training than on "bodybuilding." The two use many of the same exercises, but the focus is different, so the two approach things like the number of sets/reps very differently. Bodybuilders typically do a lot of isolation exercises for high volume (lots of sets and reps), while strength training tends to be more focused on lower reps at higher weights and on moves that involve your whole body: squats, deadlifts, bench presses, bent-over rows, and the Olympic-style moves like the snatch and clean and jerk.
One nice thing about strength-oriented workouts is that they can be a lot shorter than bodybuilding workouts; the latter can easily run a couple of hours or more, compared to 45 min. to an hour for a strength workout.
The good thing about concentrating on strength pre-T, IMHO, is that you're not gonna bulk up much without T anyway, and the stronger you are when you do start T, the more weight you'll be able to lift, and that will help you bulk up faster.
A program I like is
Stronglifts 5x5. It's short, simple, and it's worked well for me. You might also check out Elliott Hulse on YouTube (Strengthcamp). He's nowhere near as much of a jerk as he seems at first, and his approach is sensible: build functional strength and muscle size will follow. Here's one thing I learned from his videos: if you're worried about getting a big butt from squatting, do front squats instead of conventional ones. Front squats work your quads more than your glutes.
This approach is working well for me: I'm a lot stronger, I've dropped five pant sizes in about a year and a half, and after 3-plus months on T, I'm starting to see some bigger muscles, especially in my upper body.
Not too bad for a geezer, and younger guys can do even better, I think.
