Along with a relay, as mentioned above, you have to have tons of resistance. Relays screw with voltage a lot. The lack of resistance can damage an Arduino easily. I sound stupid saying this, but trust me. A lot of people damage their Arduinos when using relay because they just don't have enough resistance. Take the time to find out how powerful of a resistor you need.
My school friend just fried an Arduino Mega today in school because he had a relay but didn't have enough resistance. Stay safe.