First, turn off power to all circuits that might pass through the wall you will cut or the ceiling or attic you will work in.
Wires often pass through one area of a building on the way to another.
If you have any doubts about what's inside a wall or ceiling, shut off all power. Before starting work in a dropped ceiling , carefully lift a few tiles and look around at what's above the T-bar so that when you're working you know where ducts and wiring are. If you're cutting into a wall, to avoid damaging anything when you cut, use snips to shorten the blade of a jigsaw or reciprocating saw to an inch or so.