Flip through the different types of effects with the buttons at the bottom, then select from the named filters in the main screen.
You can choose from several standard sizes, including square shots when you know you're going to be posting to Instagram.
Bring up the onscreen controls then manually adjust focus and exposure using the wheel on the right.
Send your finished image by email, through Messages, create a Web link, or pick from social networks like Facebook, Twitter, or Flickr.
Don't forget to check out the menu before you snap your picture to toggle various useful features that will make your picture better.