It sounds like you are using the built-in API demos to try out BigBlueButton. In many of these demos, everyone is logged in as a moderator and has full privileges.
In a real session, most students are logged in as viewers and they can't make themselves presenter. Only a moderator can make someone (including themselves) a presenter.
For a better feel of how the roles work, try out our demo server at https://demo.bigbluebutton.org/ and authenticate with your Google or Twitter account. This let's you create recorded sessions, join sessions as moderator, and invite others (via an invite URL), to join as a viewer.
For more details on how the roles work, see the tutorial videos as https://bigbluebutton.org/videos/.