I wanna gain strength (for football/socder) and build an athletic body i dont have equipment. I have school bag and some chairs at minimum pls help me and Do you recommed ab exercises I'm skinny but i have no abs

I'm Ryan. I'm 12 I am insanley skinny. I am 5ft 2 or 3 I'm 32 KGS(Very underweight) I joined this Community because i wanna build muscle and not be called the "skinny boy" at school I am a strong beliver that reps dont matter form and progressive overload does! I watched the course and it said to track calories. How can i start or is there a way to track calories with a scale?