Got a used Electric oven - not sure what wire and breaker to use
You'll need:


NEMA 14-50 receptacle. 
40 ampere double pole breaker
Four 8 AWG copper conductors, or four 6 AWG aluminum conductors (Hot, Hot, Neutral, Ground).


It's common for builders to use a 50 ampere breaker and larger conductors,  to make sure the circuit can handle any range the owner's might use. But if you're installing the circuit to support a particular device, you may be able to use a smaller circuit. 

It's common to use aluminum conductors for range circuits, because it's a bit cheaper when you start using larger conductors. Type SE (service entrance) cables are common for range circuits, as it's often easier to pull a cable than individual conductors.

In layman's terms. Go to the electrical supply shop, or hardware store. Tell them what you're doing, and ask for:


A four prong range plug.
A four wire range cord.
8/3 with ground NM (nonmetallic) cable (copper).
A double pole 40 amp breaker.