To use Google to have a video chat with a friend, make sure that you and your friend both have gmail accounts.
If you both have google email (gmail) accounts, now download and install the google voice and video chat plugin here :
http://mail.google.com/videochat
This will install a plugin into gmail to allow you to use google to video chat with friends.
After you've installed the google video chat plug in, you should be prompted to restart your browser.
A common mistake people often make is to download the google video chat plugin but forget to install the plugin, so make sure you download click and instally the video chat plugin.
Once you have restarted the browser, log into gmail.
You and your friend should now appear with a video camcorder icon by your names instead of the green dot now if you have a webcam installed. (Make sure you both are NOT invisible). The camcorder icon means you both are ready to video chat with each other.
Mouse over your friends name and a menu should appear to the right of the mouse showing their name and options of what you can do with them (like email, chat, video&more)
Now YOU select "Start video Chat". Your friend should see a chat window pop up with a video and hear chiming/ringing. You have now initiated a google video chat with your friend.
hope this helps :)