Dips aren't easy, it's best to start with this variation of the dip with your feet on the floor providing assistance and decreasing the weight, and with handles on the sides of your body to decrease stress on the shoulder. 

This is similar to the first progression except performed on a bench or bar. It requires your hips to go in front of your shoulders, increasing the stress on your shoulder. 

This is the full dip

A straight bar dip is similar and different from regular dips. Straight bar dips require more forward lean which focuses a bit more on the chest, and requires significant core control as you must bring your legs forward as you lower yourself to keep yourself balanced.