Press the Xbox button to open the guide, and then select Profile & system > Settings > General > TV & display options. Select Refresh rate > 120 Hz. Note If you don't see an option for 120 Hz, select 4K TV details under Setup for more information.

It also supports 1440p and 1080p games up to 120Hz, and you can't go wrong by getting a cheaper monitor with a lower resolution either, but that means you won't be making full use of the console. Most monitors work well with the Xbox, though, so you'll be happy with any option you get for your budget.