Since a person's shoulders are made of several different muscles, you will need a few different excercises to do. Also be very careful, I know several people who have torn tissues inside their shoulders which was very painful and took forever to heal, which put a big roadblock in their gym time. Believe me, getting hurt when you're trying to keep up with a steady workout routine, really throws you off.
Consult the videos on this realjock website, and pay very close attention that you are doing it properly. On the main page, scroll down to the part halfway down that says "Workout and Excercise Finder.
" Click the drop-down box that says "Choose a muscle group to view excercises
" and select "Shoulders
" at http://www.realjock.com
Do a startup shoulder workout of 3 basic excercises: for the front, side, and back of shoulders with light weights:
Front and Side: http://www.realjock.com/article/977
The "rotator cuff" part of a shoulder is one of the most common injuries that sometimes requires surgeries to correct.
So, you've probably noticed on thinner guys when they have no shoulder muscles on the front of their shoulder, none on the side of their shoulder, and the back of their shoulder, and even none on the top part that goes from their shoulder to their neck, so their shoulder doesn't have that strong rounded out look all the way around from front to back. View excercises for each of these sides of the shoulder. Start out with very light "baby" weights less than 10 lbs so that you can do them properly, without tearing some of those tendons. Remember that those muscles and joints are smaller than other muscles in your body, so obviously you don't start with the same heavy weights as a bench press or curls.
Always make sure to eat the amount of calories and protein your body needs. You can't grow muscles if you don't feed them.