13 is very young to start aiming to build muscle.
I would focus on a sport. Train for performance in that sport and not to specifically build muscle.
learn about nutrition and healthy eating habits. Learn to love food, good clean healthy food with lots of fruit and veg.
Do some bodyweight stuff, chins, press ups and sit ups.
Spend less time on social media and more time outdoors doing stuff, climbing, kicking a ball around, running around.
Do all that and you will massively ahead of your peers in my opinion.