Welcome to Android Central! Are you accessing My Yahoo on your tablet browser? Or the Yahoo app? Remember that My Yahoo on your tablet browser might not look the same as on your desktop, since it might default to the mobile version.
What browser are you using? The command or button press to make it a bookmark varies with the browser. Look for a star on the URL bar or in the menu, or some command to "Add Bookmark."
The only problem with setting it as a homepage is that the stock browser doesn't have an actual Home button. The only way the homepage is accessed is if you open a new tab. Kind of an odd oversight.