If you could go back to day one of bodybuilding/training and start over with the knowledge/experience you have today, what would you do differently and why?
not consume protein supplements.
everything is a learning process, I made huge gains in my first year of working out so I wouldn't take that back. I'd scale back time spent in the gym though.
Oh, and I'd take back that deadlift workout I did following a soccer game when I tweaked my back

Still suffering from that shit years later.