Ok, im 13 years old and im looking to gain muscle, especially in biceps and core.?

Wrestle. After the wrestling season not only will you be in the best shape of your life (if you make it through the season) but you will also have some pointers in self defense.

During the off season lift weights and run regularly (as in at least 3x a week). Every night start doing as many push ups as you can in 1 min. And then do the same for sit ups. Do it for about 2 minutes for both sit ups and push ups (i.e. 1 min as many push ups as you can, then 1 min of sit ups and then do it over.)

Press ups. As many as you can, then one minute rest, then as many as you can, then one minute rest. Do that for like 10 or 15mins 2 or 3 times a week. The same with sit ups

play sports like baseball or tennis or a sport that uses arm muscle strength and every night lift weights for lyk 20 mins then go for a run while lifting weights for like 20-30 mins or do pull-ups, sit-ups, and push ups or something then after eat meat i guess