Do I just take power from one leg and ground? If so, how do I ground the outlet?

No, you reidentify 1 hot as a neutral, place on neutral bar, and ground is ground, place remaining hot on breaker, may have to use a short #12 pigtail to get wire into Breaker, and also pigtail #12's at receptacle to get around screws.

Maybe I didn't clarify myself. Coming from the meter are 3 big wires going into a 200 amp box. 1 wire goes to a ground bar, which is grounded to the breaker box. The other 2 wires go into breakers. They have 120 vac on them as measured from the ground bar, or 240 vac measured from each other. This is a sub box and only has 2 20 amp breakers in it. The large wires continue on into the house for the main breaker box. I want to take 120 vac from this sub box. Can't I just pigtail off one breaker and the ground bar for a 120 vac circuit (to be used for light duty outside lights)?

The "ground bar" you refer to should be the Neutral bar. This should already have the two white neutrals and green or bare equipment grounds connected for the two 20 amp breakers/circuits in that panel.

If this is the case, then you can install the new circuit exactly as the two existing.

Thank you both very much for such prompt and caring attention to my question!