Take your mirror w/you to a (Picture) Frame Shop. They'll set you up. If it turns out that when you hang the mirror it seems to be 'tilted' downwards, just put a 1/2" or 3/4" or even a 1" piece of 'something behind the BASE of the mirror. Start w/a piece of newspaper, or something rolled up until you get the mirror to 'reflect' perfectly straight ahead when you stand in front of it. You may need 2 'blocks', or something (1 for 2 'sides' of the base) to keep it from 'tilting' sideways. Know what I mean?
Be sure to try and find a Wall Stud (a 2 x 4) to nail your nails into. If you can't do that, be SURE that you use wall hardware that will support the weight of the mirror.