This might be tough if you have no knowledge of this.  Easy to do, attach a plug to the light and run it to the nearest recpitical. If you need the box in the wall,  you will need power. decide where you want the switch box. install. come out of the nearest box to the new switch box. It can be another switch box or a recptical. if it is a recptical, tie the white to white, black to black and ground to ground. If from another switch box, same except tie the black to the power black. You will need to pigtail off this. Run the power wire the to new switch box mark it.  Install the box in the wall for the vanity, run a wire from it the the new switch box. shoud use a 14/2 wire if on a 15 amp circuit, or 12/2 for a 20 amp circuit. wire in the light. black to black, white to white, ground to ground. The light will use a green or bare wire for the ground. In the switch box tie the white to white. pigtail off the gound as you will need to ground the switch also. their is a green screw on the switch for the ground. tie the power black wire [wire coming from the old receptical or old switch box, [the one you marked]] to the top screw, tie the black wire coming from the light to the bottom screw of the switch. Turn back on power and it should work.  If running power from a receptacle. Not a good idea to run from a GFCI or any other  on a GFCI circuit. It will work, but the problem with that is any fault in the circuit, such as the ground touching something, or loose caps will trip the circuit and make you think you have a bigger problem. If you have trouble you can send contact info to me and I will be glad to talk with you.  Hope this helps and does not confuse you