I don't know of any specific tutorials, but I'd work on simply understanding the basics of AJAX...
w3c's tutorial/intro, for example.
The text area (or any other visible HTML object) appearance can be controlled completely by CSS.
I recently wrote a simple "post" with it, which you can see/use
on my website, and with that, if you view the source, you can see everything that goes on.
An excellent utility to have when working with any Javascript is a Firefox plugin,
FireBug. It lets you see every javascript request, including the http requests/return values... I couldn't have found some simple bugs without its help.
As for updating whenever the database is changed, with my "post", what I have it do is:
1. When the server script is called, it checks the timestamp and if there have been changes, return the last X messages.
2. It returns a "change" value each time (0/1), so I know to update the fields if there has been an update (1), and to save a little bandwidth (by not sending the list of messages each time).
Of course it's just a start, but quite a bit of fun!
If you have any specific questions/issues with it though, do ask!
PS: One issue that took me hours to figure out, when using the javascript xml parser, remember that the server-side script needs to return the content-type of XML. I didn't do that, so the javascript always saw it as a null object.