The first setting at the top of the Game Controls screen lets you choose whether or not to allow the user account to play games at all. Assuming that you will allow at least some games to be played, you should leave the radio button selected for the default setting of Yes.
Below there, you can choose to restrict access to games based on their ratings, and / or by specific program names. It is a good idea to set the ratings level for the level you deem appropriate for the user account. Click on Set game ratings and then choose the radio button for the ESRB rating level you find appropriate. At the top, you can select the radio button to Allow or Block access to any games that appear not to have a rating.
Some games may not conform with the ratings settings, or you may not agree that it is appropriate for your child. For these games, you can click on Block or Allow specific games and enter the offending applications there.