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

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

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