If this junction box in your garage is within easy reach then you can install the switch for the lights right in the jbox and no need to run additional wires to another box. First open the circuit breaker for this circuit. Then remove the wirenut that connects all the black (hot) wires together and separate the wires out. Leave the white wires (neutral) alone as they are.
Identify the incomming power wire and combine this wire, the wire feeding the receptacle and a 7" long pigtail wire of the same gauge under one wirenut. (So 3 wires total under wirenut)
Now the remaining two wires that feed your lights will be combined with another 7" long pigtail wire of the same gauge under one wirenut (So 3 wires total under this wirenut)
If there are ground wires in this box, then remove the wirenut connecting these together and connect a 7" long pigtail ground together with the rest of the ground wires. (So 5 wires under this wirenut using a red or blue wirenut)
If no grounds are in the box then connect a 7" long piece of ground wire between the metal junction box and the ground screw on the switch.
Fold all wires nice and neat into the box by bending the wires in a gentle accordian style, but leave the 2 pigtail wires sticking out through the center of the box. Now install a plaster ring that has a 3/4" rise onto the face of the junction box. Here is a link showing the plaster ring you'll need.
http://www.cornerhardware.com/index....d339d9c9c4bec1
Once the plaster ring is installed then connect the pigtails, one to each screw, of a single pole switch by wrapping the wire around the screw on the side of the switch in a clockwise direction.
Then install a plastic switch plate cover to the front of the plaster ring. For your lights to automatically go on, just leave the switch in the ON position.