This is a site for trying, training and showing spatial SQL in general and PostGIS in particular.
If you want to start at once testing PostGIS just GO!
If you want to find a special tutorial, some of them will be listed here
and then you should be able to find more by searching for "postgisonline tutorial" on the internet searche engines
Postgisonline will be presented together with http://openlayers-buch.de
at FOSS4G conference in Barcelona under the title:
"What You Type Is What You See: Comparing two web-based open source approaches to display the results of spatial SQL queries"