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1) All sub-panels require a 4 wire circuit, 2 hots, 1 neutral and 1 equipment grounding conductor.
2) A sub-panel is both 120 and 240 volts. A neutral wire is required.
3) If you only want (1) one 120 volt branch circuit, you can use one of the black wires as the hot, the 2nd black wire will need to be re-identified as a neutral wire using white electricians tape at each end and you would need the ground.
4) You would also need to use a 15 amp single pole circuit breaker in the main electrical panel and pigtail some 14 AWG copper onto each of the existing black wires in order for the conductors to fit onto the 15 amp circuit breaker and the receptacle screw terminals.
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