I want to wire a switch from the switch to the light. There is a 12/2 wire from the breaker to the switch. How do I wire this to the switch and then wire the light which has a 14/2 running from the switch box to the light box. The wires are run, but not connected each has a green, black and white with the exception of the light, which has only black and white. Thank you!

The white can go directly to the light, only the black needs to run to the switch. Power comes into the switch via black, when you throw the switch it sends power to the light so you have only one wire coming into the switch and one leaving. The greens are likely grounded to the box and each other. Are you running conduit or cable?

Since part of the circuit has #14, you must replace it to retain a 20 amp breaker. It might be easier to replace a 20 amp breaker with a 15 amp one. Note, once you take the cover off the box, the incoming feed is still hot even with the main breaker off.

Connect all the whites together. Connect all the greens or bare wire together, to any metal boxes or housings, and to the green screw on the switch. The blacks go to screws on the switch. The push in terminals are less reliable than the screws.