The shoulders are called deltoids as in delta, because of their three sides, front rear and side. To see quick results draw out the side or lateral muscle for width. Hold two light dumbells at hang in front of you, plates touching at the edges, and raise your arms to the side up to about ear level. Tip the front of the dumbell slightly down as if pouring water out of it. Your elbows will be slightly bent, arms out to the side slightly in front of the body. Your body will look like a cross. One set of 8 or 10. 
For the back lay on a bench face down and raise the dumbells from under the bench out to the sides, again making a cross with the body (like the guys on the rings in the Olympics). For the front, raise to the front, keeping the dumbells parallel to the floor. No cross here, both arms will be facing forward straight out from the body. If you go much more than ear height the trapezius comes into play as well as the deltoids.  
Any of these can be done one handed (bend over 90 degrees while standing for the bench exercise) and hold on to a high rack or counter top with the free hand to stabilize the body. The one handed versions help take strain off the lower back if that is a problem. Eight to ten reps one set for all three versions. 
Start with a weight lighter than you think you need for each of the three(may not be the same weight for all three). You could also start with only five or so reps per set and work up, to avoid sore shoulders or injury. You could also do 3-4 reps in each direction without letting go of the dumbells. With weights less is more, and go easy at first. Make certain the dumbell collars are tight. The standing press is a good overall exercise, but doesn't get the back of the shoulders all that well. Once you get used to all this you could do the presses one set as finishing exercise after the other three. You can also start with just the pouring water exercise at first. Again, go easy at first, the shoulder muscles are relatively small. Twice a week should do it.