Take a look at the help file on Paint.NET by pressing the "F1" key whilst in Paint.NET, it should provide the main basic information, alternatively go here. Take a look at the stickied tutorials in the Tutorials page, and also download the plugin packs (they will be stickied) in the Plugin page, and you can also download a few more separate ones too if you wish. Lastly, all i can say, is experiment with what you do! Try out new settings, plugins, blend modes, etc. and gradually you'll become much better!
I'll look forward to see what you can make.
P.S That was probably the longest post I've made here so far.