I log in to my Yahoo account. But after I go to empathy, it just says that "No online contact." How can I chat?
This may vary depending on your window manager.
Go to the top right of the screen and click on the little envelope icon. From the drop-down list choose "Chat". A little window called "Contact List" should open up. (You may have to click on a blue and white word balloon in your task bar.) On the top left of the screen it should say "Empathy Internet Messaging". If it doesn't do this click on the "Contact List" window. Hower your cursor over "Empathy Internet Messaging" and you should get a menu. Choose Chat => Search for Contacts.
Ubuntu likes to hide it's menus, but once you know where they hide it's not a problem.
Hope this works for you.