Starter will not engage when turning the key on

Does it fail to start or crank, JUMP started?
does the rested battery read 12.6v exactly or more (warm day)?

battery dead? charge it.
battery cables terms filthy. (corroded)
Not in park? got A/T or M/T? can help not knowing tranny.
not holding brake pedal down (USA? where are you, 1000s of posts and not one post says, hey im in Canada or?
try wiggling the gear shifter.
try wiggling the brake pedal (harder)
try hitting the starter, yes, this can wake it up.
the starter is bad, you can hot wire any starter to test in on car, ask.
the key line that actives the starter is dead, ask how to test that.