Apple's AirPods have a simple one-tap setup process that syncs them to any compatible device through your iCloud account. Your AirPods should always be on and connected to your device after the initial setup is complete. Your device needs to be running iOS 10 or later and is also needs to be connected to your iCloud account. You can check what iOS your device is running by navigating to Settings > General > About. From here you can see what version of iOS you're running in the Version section.
You can see what devices are connected to your iCloud account by navigating to Settings > iCloud and tapping your Apple ID name. From here tap Devices and answer your security questions if prompted to do so. If your AirPods are connected to same account as your iPhone and iPad, but still do not work, try charging them.