The single most important thing you can learn to do is squat and deadlift with good form. Reps of 3-5 and as heavy as you can go whilst maintaining good form. Do this twice a week (squat one day deadlift the other) then on the other gym day do your bench press, chin ups etc.

Stressing the whole system under significant load (safely) will force you to fill out. Don't go crazy on the calories, the body can put down fat much faster than muscle. You just need to create a sufficient demand for calories to be partitioned to lean tissue.

Eat good natural food and be healthy.