The very simple way to add mass:
eat more calories than you need. Since you are new I'd suggest 0-500 more calories than you burn. more than that it will be fat I think.
Sleep a lot. If you are sleeping 3 hours that won't work. If you sleep ~8 that's pretty decent.
Train every muscle hard at least twice per week, preferably 3 times.
The exercizes that in general are thought to be the ones you should be doing are :
back squat, deadlift, bench press, rows, pull ups, dips, overhead press.
all of these except deadlifts should be done twice per week(squats maybe 3 times. You said you won't do squats, if that's the case do leg press but it really isn't anywhere near as good as squats) Also you should do 3-5 sets of 5 reps on all of them except deadlifts which should have 5 sets of 1 rep.
I also personally suggest adding some lower back and abs exercizes every day. Either bodyweight or very light weights up to 40kgs/~85lbs. these should be done for 2-4 sets 5-20 reps depending on weight.