Sarah...........Visit an Auto Parts Store and get a package of rear view mirror adhesive.....also carry the mirror into the store in case you need to purchase an allen wrench to remove the small mounting base........
READ the instructions that are included on the package.........
Mark where the mirror 'was' on the outside of the windshield.........(You can skip this part but you'll appreciate it later when you're sitting there with the fresh glue and the clean windshield.......something is going to be off-center or crooked or too high........)
Generally you need to clean the glass and the base/mount......with Razor blade and solvent........
Then apply the glue to the base and hold in position for a minute or two (position using the marker on the other side of the glass)......"Right-side Up"...........then re-attach the mirror to the mounting base..........