There are three main ways to start them working:
You can go to the Google Hangouts page and click the red Start a Hangout button on the right-hand side.
If Google is your home page you can click the Hangouts symbol there and then click on the 'Hangouts Invite' section.
The same symbol should appear under a post on your home page, so you can also click that, which will direct your friends to the post.

When you click any of the above, you should get taken to a screen where you can check your video settings (the feed is at the foot of the page) and invite one person or a group to join you - you can learn more about this here. You have to remember to click the green Invite button or they won't be able to see you.

To prevent under 18s from accessing the hangout, you need to tick the 'restrict minors' box.