This is part of our massive Apple Watch FAQ, which should tell you everything you want to know about the gadget, its features, and its interface. You can find it here
Yes! This is the oldest iPhone that the watch works with. An iPhone 4S, for example, is out of luck. But the 5 (as well as the 5C, 5S, 6, and 6 Plus) works with the Apple a Watch.
Just make sure it's on the latest firmware so you have the Apple Watch app installed and you'll be ready to rock and roll come April 24th.