What’s included
To use your iMac, you need two accessories included in the box—the Magic Keyboard and the Magic Mouse 2.
Important: To begin using your Magic Keyboard and Magic Mouse 2, simply make sure the on/off switch of each device is on 
(green is visible). The devices connect to your iMac automatically.
Magic Keyboard: Your rechargeable Magic Keyboard is already connected with your iMac and is ready to use. When you need to charge the keyboard or if you need to connect it again, connect it to your iMac using the included Lightning to USB cable.
To customize how your keyboard works, in System Preferences, click Keyboard, then click the buttons at the top to see the options available for keyboard, text, shortcuts, and input sources. See iMac Magic Keyboard to learn what the function keys on the keyboard do.
Magic Mouse 2: Like the Magic Keyboard, your rechargeable Magic Mouse 2 is already connected with your iMac out of the box; you can use it anywhere within range of your computer. When you need to charge the mouse or if you need to connect it again, connect it to your iMac using the included Lightning to USB cable.
To customize your gestures, in System Preferences, click Mouse, then click the buttons at the top to see all the scrolling, clicking, swiping, and tapping options.
If you configured your iMac with the optional Magic Trackpad 2 or Magic Keyboard with Numeric Keypad, it comes connected with your iMac and ready to use. If you purchased a Magic Trackpad 2 or Magic Keyboard with Numeric Keypad separately, connect it to your iMac with the included Lightning to USB cable to connect with it using Bluetooth. Use this same cable to charge your devices as necessary, then unplug them and use them in the location that works best for you.