Once you are familiar with the basics of Paint.NET, you can move on to the Tutorials section to see how some things are made.
http://paintdotnet.12.forumer.com/viewforum.php?f=15
When running some tutorials, you may need to install some plugins. This page explains how to install plugins:
http://boltbait.googlepages.com/install
Here is a list of plugins I've created:
http://boltbait.com/pdn
And, you can always find more in the Plugin forum:
http://paintdotnet.12.forumer.com/viewforum.php?f=16
Then, once you have created your own masterpieces, post them in the Pictorium:
http://paintdotnet.12.forumer.com/viewtopic.php?t=1072
Also, be sure to read the rules:
http://paintdotnet.12.forumer.com/viewtopic.php?t=3446