I have 3 wire ( black, white,green) 240 on a double pole breaker. White and black wire connected to breaker green to neutral bar. Want to convert to 120v single pole breaker. Which wire do I connect to new breaker? What do I do with the other wire?
You don't need to change the breaker. Just move the white wire from the breaker to the neutral bar. The other half of the breaker will be unused.
And change the receptacles to NEMA L5-30 or TT30.
You cannot put normal, common NEMA 5-15 and 5-20 receptacles on a 30A breaker. In fact, 30A 120V circuits are almost useless, except for small campers using the TT30.
If you need to put common receptacles on this circuit, you must change the breaker to 20A. (or 15A if there is only one socket and it's 15A).
If you need 30A capacity but need to feed devices with standard 15/20A plugs, then you need to fit a 120V-only subpanel, with all the rules for that (e.g. if it's in an outbuilding you'll need a main shutoff and grounding rods). It can then serve 2 or more 15 or 20A circuits. In fact, that is how my house is wired.
Neutral is always white (although white isn't always neutral)
Since you're converting this from a hot-hot-ground (240V-only) to a hot-neutral-ground (120V-only) circuit, you'll need to move the white wire to the neutral bar to serve as the new neutral. The black hot wire lands on the new breaker's load lug; make sure to have the breaker switched OFF when you are connecting/installing it, by the way, and also to use an inch-pound torque screwdriver to torque the lugs to the marked tightening torque.
Black wire would go to the new 1-pole breaker. White wire would go to the neutral bar. Green wire to the ground bar.
What receptacle do you want to use? Do you want to make this a "standard receptacle" or a special receptacle?
What size is the conductor in this wire? If it is larger than #10, you might have trouble connecting to a standard duplex receptacle so you might pigtail on 6" long pieces of #12 copper. A 20-A receptacle would give you more flexibility for connections.