There are two ways to set your audience to Public:
-From your News Feed, click your name in the top left.
-From your profile, click the three dots in the top right, then click Edit Profile.
-Click your audience next to each piece of information, then click Public.
If you want to share something with the world, set the audience to Public. When you do this, anyone - even people who aren't on Facebook - will be able to see what you've shared.