i have a macbook air with macos mojave. what is the best way to use my ipad as a second monitor

You will have to upgrade to macOS Catalina to use Sidecar, which uses the iPad as a second display.

more info on sidecar here:

https://support.apple.com/en-sg/HT210380

If your Mac isn't compatible with Sidecar, you can use Duet: https://www.duetdisplay.com/

~Lyssa

I use Yam Display https://www.yamdisplay.com/ works well