Well, I do not claim to be a well expert but I have worked on my own a good bit and seen it worked on as well. At only twenty feet, you can use a jet pump if you'd prefer. This site has a good discussion (and diagrams) of jet and submersible pumps.
Water Well Pumping, Well Pump Diagram, Well Pump Prices
Both systems work in the same way once the water gets to the surface. As the pressure tank fills, air in the top of the tank is compressed which produces pressure on the water. Once the pressure rises to a certain level, say 50 pounds, the switch senses this and will cut the pump off. As water is used, the pressure drops until, let's say at 30 pounds, the pump is switched back on.
Most diaphragm tanks have a single opening in the bottom of the tank. You will need a brass "T" fitting which will have the water entering from teh pump at one end and exiting to your house at the other end. The switch will also connect at one end or the other of this fitting.
I'll answer any questions I can. If I can't, I will be honest and just tell that to you. There are others on this site who are knowledgeable.