If you are near home depot or lowes................,,,,,,,,,BUY A PLASTIC CUT IN BOX(THEY HAVE FLAPS ON THE SIDE) PLACE THE BOX FLAT ON THE WALL ( THE LOCATION YOU WANT THE NEW OUTLET) USE A PENCIL TO DRAW THE OUTLINE OF THE BOX. Buy a sheet rock saw blade( it has a handle) and cut the outline you just drew..Run a piece of 12/2 romex down the wall(inside the wall) shut off power to the outlet you will tie into , run the romex to the existing box ( old outlet) splice
Color to color all whites tie together/all green spliced together/ all blacks tied together. Run that 12/2 romex into your new box......The new cut in box will have 1 green wire
1 black wire 1 white wire (remember you shut off the power to the old outlet) the black wire goes to the brass screw the white wire goes to the silver screw,, the green wire goes the ground of your new outlet.You can ask the store how to install the cut in box...Turn on power to your new outlet