There are several ways to open a chat buffer. The shortest way is to put point over the person you want to chat with in the roster display and hit RET.
You can also use menus to access commands. In the roster display, you can access several menus through keystrokes or mouse clicks. You can bring one big menu up by pressing the second mouse button, or you can bring up the “chat menu” by typing C-c C-c. If you do the latter while point is on a roster entry, that entry will be the default value when you are asked for whom to chat with.
You can also use the function jabber-chat-with, which is what
the menu item is bound to.
Now, try opening a chat with someone. A buffer named
*-jabber-chat-:-person-* will be created and selected.
Type your message at the end of the buffer, and hit RET to send
it. To include a newline in your message, use C-j.