For a second there, I got all pumped to hear Steam Family Options had gone live for all users after previously being restricted to beta participants. Then I realized this isn't the same thing as Steam Family Sharing -- rather, it's a set of parental controls. Still important, admittedly.
To enable this feature, head into Steam's Settings menu, which will have a listing for Family Options under the Family tab (or, if you're in Big Picture Mode, simply hit the Family Options button). From here, you'll be able to set up a PIN that can restrict access to the storefront, specific games, community functionality, and Steam Friends.
Once you know where these options are located, the process is fairly self-explanatory, but Valve has included further instructions at the link below just in case. The company's example of preventing "kiddos [from] going crazy with the credit card" is all-too real.