Hi, I would like to know about what to do in the gym. Im on school vacation now and this is for 3 months. So i have all the time i need to go to the gym. I want to loose a few pounds, but at the same time gain muscle... What should i do..?

Some people told me to keep my HR in 136 for fat burn and 166 for cardio... But should i use that or what..? by the way, im 15, weigh 120lb and am like 67in.
my BMI is 19kg/m2... so what should i do to gain muscle and burn a little fat.

Thanx