Drink a gallon of whole milk (work up to it, if you have to) each day, on top of whatever you are currently eating (and hopefully you are currently eating a good bit).
And I wouldn't do anything but compound lifts, 3sets, 5 to 8 reps. 3x/week. Maybe an additional short workout 1x/week for core. And don't do any cardio other than what you have to for sport-specific work (eg, basketball, boxing).
Keep it extremely simple, but extremely effective. Diet + Recovery are the two big keys (and obv going hard at the lifts when you do them).
I've never met anyone, even ppl who could "never gain", not gain tons of quality size by doing the milk + above.