Most aquarium heaters have a built in thermostat, that will heat when the temp drops 1degree below the set temp and stop heating when the temps hit 1 degree above the set temp. So get one from Eheim, Fluval, Aqua One, Aquael, Hydor, to name a few brands.
Depending on where you are from.
A temperature controller like the one kms posted is good as a backup protection for the built in thermostat in cheap aquarium heaters. And it can be used on heaters that do not have built in thermostats.
Just make sure you buy one that is a suitable size for your tank. A 25W or 50W will be enough.