1. You can directly click on the hangouts link given just below the status updates or shared posts on Google+.
2. You can also have hangout from the messenger itself. Just click on the name you want to hangout in the chat list, It will open a chat window. In that chat window on the extreme top left you will see a hangout logo, just click on it and you can enjoy hangout.
Note: For hangout you need to install Google hangout plugin. You should also be equipped with a working microphone and web/cyber camera on your laptop or desktop set.