To attach a ground wire to a metal box:
If it is a relatively recently manufactured box, one or more of the holes in the back of the box is threaded for the purpose of screwing in a ground screw. Wrap the end of the bare wire or the stripped end of the green wire around the screw and tighten.
If the box is old, it may not have a threaded hole for grounding. In that case, use a ground clip, which holds the ground wire tightly against a side of the box.
Either way, I am not referring to the thin bare metal wire that runs through armored cable.