I have a pull chain light in my shed. Power comes in to the light. I want to add a switch to the light so I don't have to use the pull chain. Is it possible to run electricity down to a 2 gang box and have a switch that controls the light and a receptacle that has constant power?
Very easily. You need to run a three wire cable from the light fixture to the location of the switch/outlet. Disconnect the hot lead to the fixture and connect it to the black wire in the new cable. Connect the red wire in the new cable to the hot terminal or wire on the fixture. Connect the neutral (white) wires in the fixture box. Connect the ground wires.
At the switch box, connect the black wire to the outlet and to one side of the switch (simple switches don't care which side, but some advanced switches have a LOAD side). It's often easiest to connect the incoming black wire directly to one hot terminal of the outlet and a pigtail from the other hot switch terminal to power the switch. Be sure not to break the connecting tab on the outlet between the terminals. Then connect the red wire to the other side of the switch.
Connect the neutral wire to the outlet, and to the switch if it is one that calls for a neutral.
Connect the ground to both the switch and the outlet. Turn on power and you should be done.
The type of cable you need depends on both where you run it and your local code. You may be able to use simple non-metallic cable, or you may need armored cable or conduit.