OSM – PostGIS – Mapnik problem!

I have to learn how to make maps using OSM data. The data will be in held in a PostGIS database and my role in the project is to learn how to take this data and make pretty maps using Mapnik, store the tiles and use something to make slippy maps that can be accessed by the public for free via a web server. In order to fully understand / get to grips with my end I thought I had better learn / get familiar with the whole process. To the end I have done the following:
*I have installed the OSGeo live version 5 bundle on a virtual machine on my PC. I believe this means it is running in ubuntu.
*Then downloaded United_Kingdom.shapefiles.zip, unzipped and used QGIS to make the shape file go into an sql database (PostGIS).
*I think I then access mapnik through a terminal thing a bit like when you want to run command lines through dos. There doesn’t seem to be another way that I could see? I am sorry if I sound like a bit of an idiot, but I don’t really understand what Mapnik is, I thought it was going to be something like ArcGIS or Mapinfo, both of which I’ve seen before. Anyway, I followed some instructions using something they called “python bindings” and managed to get my .shp files to make maps in the .png format. I have some instructions on how I can make that ‘executable’ – whatever that means?

But I need to be able to make these maps from data in the PostGIS database, and I can’t seem to figure out how to do that. I have tried some code copied off OSMwiki, but that didn’t work. Someone has suggested that Mapnik maybe doesn’t know how to read from PostGIS and told me to type ‘registered datasource : postgis’ but I got ‘registered : command not found’ and I don’t even know what that means!
I have been using this OSGeo thing as it came; I have not installed or uninstalled anything else. I am sorry if I am asking help on something really basic and potentially obvious, I don’t come from a computer background and I think I am out of my depth!!


Maybe at http://switch2osm.org/ there are some hints how to process your data?