The shoulder makes the arm I always say, so in order to get that nice round shape, you have to hit all three head of the shoulder. Shoot for 15 reps and pick a weight where you begin to struggle around 12 reps. If you need to rest a moment to finish them, that's ok. Muscles grow when you push them so if can do 15 reps no problem, there's no reason for them to get stronger. When 15 reps gets easy it's time up your weights. There are lots of different shoulder moves, but this is one of the best basics to have. Good Luck!