The key to good approach shots is:
1. Choosing the right club and Power/Spin/Line
2. Executing it properly.
To get the right club you need to take the following into consideration.
1. Your lie (are you in rough/sand ?)
2. Line to the hole (are there trrees in the way??)
3. Distance to the hole
4. Is the green on a drop or elevation? If it is elevation you need to add more distance to the club to get the carry. But remember when the ball comes down it lands higher up in its trajectory so will roll farther. The opposite is true for drops in elevation. The ball will land much more perpendicular to the surface, so will roll very little
5. Wind. The wind will add or take away distance. It will change the line. Cross wind appears worse if you dont hit an "excellent' shot. 9MPH appears to add or take away about 5 yards.
6. The green. You need to know the green. For example 11 on Bethpage has a huge ridge on the green. I always add 5 yards to my approach.
7. Know your clubs. Know what they do, how far they carry, what spin does to them etc.
Lets start with that and then let me know if you have any more questions.