Where do I put the hot wire on a double pole 20 amp breaker?
What are you trying to accomplish?  

A double pole breaker is used to supply either a 120/240V circuit, a straight 240V circuit, or a 120V multi-wire branch circuit. 

If you're wiring a 240V circuit, the black wire goes to one terminal and the white goes to the other. If you're trying to wire a 120/240V circuit, or a multi-wire branch circuit,  you'll need another wire. If you're trying to wire a 120V circuit, you need a single pole breaker not a double pole.

Both terminals on a double pole breaker are hot.