What kind of adapter do I need to connect this external monitor?
If you register your Mac with iCloud / AppleID and then use the iOS Apple Store app - it shows compatible adapters. Two I know that work well are the Apple USB-C Digital AV Multiport Adapter adapter and another is the Belkin USB-C to HDMI Adapter

https://store.apple.com/xc/product/MUF82AM/A
https://store.apple.com/xc/product/HKQ22ZM/A

Monoprice also has great value and quality adapters in general, but I’ve not tested these specific ones yet.