Hi Abbs
As skeletal muscles work in opposing pairs of groups, you need to look at the chest and the upper back - both of which work with/against each other to keep the shoulders in the correct place. I would advise exercising both and stretching both, personally, but to answer the question, it's likely the chest is too tight (so needs stretching)and the shoulders/upper back too weak (so need exercising)...
So the muscle names:
Chest:
Pectorialis Major
Pectorialis Minor
Subclavius
Upperback/shoulders
Trapezius
Rhomboideus muscles
Levator scapulae
If you want to get complicated, you could look at all the muscles that move the shoulder blade, eg. the rotator cuff muscles, but I think for your assignment the 3 above is probably enough.
Exercises: (find some pictures on the internet for your assingment)
Shoulders/upperback:
Prone rowing with hand weights (ie lie face down, with arm off the couch, with a dumbell, pull the hand away from the floor, up to chest level)
Seated rowing - on a rowing machine/dynaband/weights machine
Seated shoulder press (lifting weights above head)
Seated lateral raise (elbows at right angles, against sides, then raise out to sides)
Chest:
Press-ups, on floor or against the wall
Inclined chest flies
Inclined dumbell press
Stretches: (find some pictures on the internet for your assigment)
Shoulders/upperback:
[ul][*]Interlock fingers with straight arms, push down, looking at the top of your hands, chin on chest, get a really good stretch to your upper back.[*]Hold your head with one hand and opposite shoulder with the other hand on the same side, pull your ear to shoulder. Good stretch for neck, Repeat both sides. Repeat this tilting the head forward and to the side, to get a good stretch to the Levator Scapulae.[*]Use a doorway to hold your arm in place, turn the body and get a good strecth to the back shoulder area.[/ul]
Chest:
[ul][*]Repeat stretch 3 above for the chest, turning the otherway.[*]Link hands behind, raise chin, to give a good stretch to the whole front upper torso.[*]Walk through a door way with both arms open to the sides, lean forward as the arms get stuck behind to give the chest a good stretch. Repeat this with arms at different heights to stretch the full pecs major.[/ul]
Hope that helps a bit...