If you don't already have one, get yourself a Jabber/XMPP client. Here are some suggestions:
platform suggestions win32 'nix macos
When you start the client for the first time, it will ask if you want to register an account on a free public server. You can access the conference service from any server on the network, so go ahead and register an account with the server of your choice (the jabber.org server is a good option, but there are many public servers on the network, or you can run your own).
If you want to find out more, instead of choosing these defaults, here are pointers to some additional information:
list of clients: http://www.jabber.org/software/clients.shtml end-user howto: http://www.jabber.org/user/userguide/ public server list: http://www.xmpp.net running your own server: http://www.jabber.org/admin/
To make sure everything is running ok, do "Join Group Chat" (or the equivalent) with your client:
Room: ecrit Server: jabber.ietf.org
This conference room is up and running right now, although probably no one will be in it when you connect. (Note: you may need to provide the full conference room address, which is firstname.lastname@example.org).