You will come across this side quest during the Seek 'N' Loot main quest. You need to disable the electricity blocking the gas station.
Each power station you need to disable will be marked on your mini-map. The easiest is Station B which you can walk right to.
Switch A is a little trickier, you must use jump up the truck on the left side of the building to get over the fence. There are two Thugs and and bunch of other zombies so be careful.
Switch C is a ways away. When you get near it you'll have to find a way up a ladder that is just out of reach. Stack the crates to reach it. You'll complete the quest when you turn off the last switch.