Can I use 14 gauge wire to hook up two spot lights on both sides of the garage. Power is off a light swith wich has 12 gauge wire

#12 wire will indicate it probably is a 20 amp circuit, therefore the new extension to the circuit must be #12.

Check the breaker rating.

That and never assume that with the switch alone being off makes it safe. Kill the breaker and read the rating on the breaker. Homes get "fixed" by every owner who dabbles in electrical work like you are doing right now. Never assume anything.