Limit screw is the first thing to look at. Next, allignment of the derailleur cage, then height of the derailleur above the cogs.
It's also possible, the cable is too tight to allow the derailleur to go all the way down to the limit screw setting.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.