You dont lock the grid to a position, you lock the cameras translates rotates etc select them in the channel editor right click over them and go to lock selected, when youve lined up the grid with something in the picture (to get a point of reference)
Also to get the correct perspective you migh have to adjust the cameras properties to match the camera that took the picture as i'm hazarding a guess that youll want to know why some things dont appear to line up correctly when you start to model them in the image. YOu can "fudge it" playing arround with the settings.
Have a look into camera projection as that should help you out
"No pressure, no diamonds" Thomas Carlyle