How do I start/join a G+ Hangout?
Make sure you have updated to the newest version of the G+ app.  You should have version 2.3.1.242969 that came out yesterday.

From there you need to go to messenger and find the person you want to hangout with and send them a message.  Once you send them a message you will see a hangout icon appear in the top right corner.  See here for more information.