If it’s a brand new Apple TV, plug it in and stop at the initial setup screen (pictured). Grab your iOS device, unlock it, make sure Bluetooth is enabled in Settings, and touch it to your Apple TV. WiFi settings will transfer automatically and get your Apple TV onto your network, and your iOS device will ask for your iTunes Store password, then prompt you to save it in your Apple TV. You’re all set.
If your Apple TV is already setup and you need to change WiFi passwords or iTunes Store accounts, go to Settings > General > Reset (at the bottom) > Reset All Settings, which will kick you back to the aforementioned initial setup screen.