Tap on the plus button in the center of the tab bar at the bottom of your screen to access the Composer. The Composer launches as a screen above the main feed, and it has its own tab bar which allows you to select the kind of content you would like to post.
When the Composer first appears, Photos is the default selection. You can change this by tapping on any of the tabs along the bottom: Photos, Camera, Location, Text, and Mood. Each of these options has its own help topic.