Well you came to the right place to learn about mapping
gotta say that posting a map or two in your first post is a good way to start off here so have some rep
We have a few tutorials here for Paint.Net, but the ideas and some, if not most, of the techniques given in the other tutorials will translate over with a bit of fiddling too. Don't be shy to ask questions, cause we are not shy about offering advice .
One thing that we are a bit picky about around here on the larger regional and world maps is the rivers. I notice that a few of your continents have water systems that span from one ocean to another. While the canal in Card works I a, not too sure about the others. Hard to tell until you put some mountains.
Looks good so far. You should start a WIP (Work in Progress) thread and we will have you finishing these maps in no time. With lots of pointers along the way.
Paint.Net is not one of the most used programs here, and it is nice to see more people using it, so if you have any pointers for any users following you, please post a tutorial or two yourself.
Like I said, this is a great resource for mapping so look around, participate, and post your maps as you go. Welcome and enjoy