my old apple Magic Mouse cannot connect to my new Mac, please help

Earlier models of Apple's wireless input devices don't have a Lightning port. To set them up, you need a wired or wireless mouse or trackpad that is already connected to your Mac. If you're using a laptop computer, you can use its built-in trackpad during setup.

1. Turn on your device. Its LED should start blinking to show that it's discoverable and ready to be paired.

• • If the LED never turns on, you might need to change the device's batteries.
• If the LED turns on briefly but doesn't blink, the device might be connected to another Mac that is nearby. On that Mac, choose Apple menu  > System Settings (or System Preferences), then click Bluetooth. Click the Show Detail button , then click Forget This Device. Or move your pointer over the device's name, then click the remove button . After removing the device, turn it off and back on.

See If you can't connect your Magic Keyboard, Magic Mouse, or Magic Trackpad to your Mac - Apple Support