What type of cable to I purchase to connect my MacBook Pro to my HDTV?

for most recent MacBook Pro models and typical TV sets, that would be a USB-C to HDMI adapter or adapter/cable.

Since some MacBook Pro models have few USB-C ports, apple provides the increased flexibility of an adapter that combines HDMI out, "regular" USB-A connector for data, and a USB-C charge-ONLY port in one [pricey] device:

USB-C Digital AV Multiport Adapter -- $69.00

Note: required HDMI cable is additional

Depends on the model/year MacBook Pro and its ports and the TV's input ports.

Apple TV hardware can also work here, via AirPlay.

Or directly, if this HDTV supports AirPlay.

How to AirPlay video and mirror your device's screen - Apple Support