Creating a simple messaging system using Smack is easy and you can build clean code too.
You can get plenty of tips on this site itself, the home page of smack itself shows you how a message could be sent using about 3 lines of code !
I would advice you to read through the documentation available on this site at http://www.jivesoftware.com/xmpp/smack/documentation/
You could leave out the roster and extensions for now. Reading through the documents should give you most of the building blocks.
I presume that you are comfortable with Java. You could go like :
1.Create two user accounts on a public jabber server.
2.Download smack and ensure that smack.jar is on your classpath.
3.Try out command line clients to connect to the jabber server and send out messages and also receive them.
4.Develop a simple UI to allow typing and sending of messages.
Hope this helps and please get started and keep posting on your progress so that more help could be provided as you go.