An Xbox One console is required for play, as well as an Xbox Gold account and an Internet connection. You can find Roblox in the Xbox Live store. It's free!
There are many different games you can play with Roblox. When you first start out you can find them in the Home screen and Games screen. After you've played some games you'll also be able to find your recently played games in the Home screen.
Each game inside Roblox on Xbox will have slightly different rules, controls, and game mechanics. The general controls will be the same however, as shown below.