First things first !! First post...first map...first rep
Nice start on the land masses.There is a slight prob with the river/delta in the north of the east continent. The large body of water that runs down the center would be at sea level, and would not have a river connecting it to the water to the north. at this scale I would first place mountains then do the rivers based on the lay of the land.
Just my two cents worth there
As for the software, there are a few tutorials in the Tutorial Forum that deal with atlas style maps. Check them out to see which one you like best and look at what they are using. We also have a Software Forum where all kinds of questions are answered on that subject. I have been surprised before to see what Paint can do, and is you want to stick close to that, you should look into Paint.Net. We have a few members who are using it and might give some pointers along those lines. (Peace-Heather. I am looking at you ).
Most people around here use one of three programs: Gimp, Photshop and CC3 (and its add-ons). The tuts for Gimp and Photoshop are more or less interchangeable, but most of the tuts are able to give pointers as to what is possible if you can figure out how to convert it to your program.
Another thing you should do is start a WIP (Work in Progress) thread in the appropriate forum and continue to post your progress there. You are bound to get plenty of pointers and suggestions as to what works and where you might have gone off track. Be not afraid, cause we don't bite.