How do you connect a mouse to your 2017 MacBook Pro

What kind? A Bluetooth mouse would need to be paired in System Preferences. A regular wired or wireless (dongle) USB-A mouse would need either a USB-C to USB-A adapter, or a USB hub or dock that has USB-A ports. Otherwise it should be pretty easy to use.

Read here: If your Apple wireless mouse, keyboard, or trackpad isn't working as expected - Apple Support