Are you planning to create a web app? Are you thinking of deploying your app on the "Google App Engine" infrastructure? If yes, start with the overview of the Google App engine instead of directly jumping to the meat of things.
If your aim is to create a simple XMPP client so that you can interact with an open XMPP service like Google chat, look into the Smack XMPP client library. Start with the Smack documentation. If case you face problems with the same, seek help on their forums.
The link I posted in my previous post contained the client library which you can use for making a gmail chat bot. Yahoo uses a different Yahoo Messenger protocol (YMP) for its messenger services and thus would require you to use a separate client library
It would require quite a bit of research if you don't intend on using any of the existing libraries and creating your own one. If you still insist on doing it, start with the XMPP specs/YMP specs for the same.