how do i get my iPad to show up on sidecar?

Does your Mac meet the SideCar hardware requirements?

Sidecar requires Mac with Skylake processors and later models, and iPad models that support Apple Pencil.

Sidecar-supported Mac models:

• MacBook introduced in 2016 or later

• MacBook Air introduced in 2018 or later

• MacBook Pro introduced in 2016 or later

• Mac mini introduced in 2018 or later

• iMac introduced in late 2015 or later

• iMac Pro introduced in 2017 or later

• Mac Pro introduced in 2019

Sidecar-supported iPad models:

• 12.9-inch iPad Pro

• 11-inch iPad Pro

• 10.5-inch iPad Pro

• 9.7-inch iPad Pro

• iPad (6th generation or later)

• iPad mini (5th generation)

• iPad Air (3rd generation)

Use your iPad as a second display for your Mac with Sidecar - Apple Support

Follow the steps in the link provided by Michel above: https://support.apple.com/en-us/HT210380

Also:

• Both devices must be signed in to iCloud with the same Apple ID using two-factor authentication.
• To use Sidecar wirelessly, both devices must be within 10 meters (30 feet) of each other and have Bluetooth, Wi-Fi, and Handoff turned on. Also make sure that the iPad is not sharing its cellular connection and the Mac is not sharing its Internet connection.
• To use Sidecar over USB, make sure that your iPad is set to trust your Mac.

Follow the directions in my provided link above.