I just turned 13 on the 23rd of June, but I have already hit puberty for a little while. I'm interested in getting better abs/core and stronger arms, but I'm not completely sure what's right for me at this age. Any advice anyone?
I'd say that the simplest advice would be to get into sports or find one dicipline and go with it. Swimming regular is a great start but anything that gets you moving regularly and consistently will help. Watch what you eat but don't 'diet', if you're exercising regularly you need the fuel.
I'd hold off on serious gym going until you're comfortable with what you're capable of. People often rush in and are then put off when the work seems too hard, so stick to what's fun for you until the gym becomes something that compliments that fun stuff.
Alternatively, ignore me and check out some of the ab workouts on here