Try tghis bodyweight routine it helped me gain eight pounds in two months:
pushups 2 sets of eight reps
incline Pushups 2x8
Decline Pushups 2x8
Pull Up negatives (If you have a chin up bar; jump up and lower yourself down extremely slowly)
Chin Ups 2x5 (If you have a chin up bar)
One Leg Calf Raises 2x8
Two Chair Dips 2x8 (put feet on chair and hands on chair; make sure to isolate movement at elbow and shoulder)
Knees Raised Crunches 2x8
Lying Leg Raises 1x8
Make sure you do all excercises slowly and with good form to gain the most uscle
Also, take in A LOT of calories to gain muscle, if you take in too few you can LOSE weight!
You want to find a good weight lifting workout and eat a good diet. Take your weight times .8 and eat at LEAST that much protein a day. You definitely want to be sure you're lifting though or it won't do mch good.