A relay is one solution - and its conceptually probably the easiest. Its a fairly regular question here,
and often people try to invent their own transistor or FET based power switch and get it wrong, because
with a transistor/FET switch the voltage is inverted (with a relay the input and output are isolated which
is simple).
How much current is the project needing in total? A small reed relay might be enough if its low, and
some of these are small enough to be driven from an Arduino pin (but don't forget the free-wheel
diode, essential for any relay driving circuit).