How to begin with ajax?

[eluser]Buso[/eluser]
I know the basics, but I would like to learn good practices.
I don't know how to make php + ajax code in a way that if the client browser doesn't have jss enabled, I can still show him the login, for example (well, I know how to, but not in a good way.. i wanna use the full power of CI and say, Jquery)
I mean I would like a link to the login form, which fades-in and out by ajax, but if jss isn't enabled, the login is still shown somehow.