My question is, how do I take a line that is wired to both terminals of a double 20 amp breaker and make it so I can use it as a normal 110 volts service for a switch. To start I found that a meter reading of 110 volts was achieved by putting the terminals on either the black or white wire and the ground. After talking to an electrition at work I was advised to take the white wire out of the 20 amp breaker and place it on the neutral bar, did not work- no power at receptacle. Then I tried adding the black to an existing 15 amp breaker, still nothing, so I then took the black wire from that 15 amp breaker and put it in the 20 amp breaker and that fixture worked. I then put the original black wire in 20 amp breaker and still nothing happened. So what do I do, is the problem the wire gauge?

Black to a single pole breaker and white to the neutral bar should work. I would recheck your work. Most breakers will accept a wide range of wire sizes. Inspectors will only accept wires as big or bigger than rated for the breaker.

Dear labman, thanks for your prompt answer. You are right of course but I'm working with a double pole 20 amp breaker. I'm wondering if it is necessary to pigtail two blacks from each terminal to my black to utilise both sides of the 20 amp breaker. Also the receptacle had no power to it when I placed my black wire on a 15 amp breaker which made me wonder if the wire gauge was too thick for 15 amps. What do you think...

You really a single pole breaker, but the double pole should still give power. Don't pigtail them like that. The 2 wires from a double pole breaker have 240 volts between them.

You have a bad connection somewhere. With the black wire connected to the double pole breaker, and the breaker on, go back and check for 120 volts from black to ground at the receptacle. Also check for continuity between white and ground.

15 Amp breaker sounds bad. Turn all the way off and back on.
Like above, you don't want to jump across both hots, Breaker will trip.