Well lets see
1. To make your own characters.
You could model them in a 3d cad and render them.
Draw them on paper and scan them then use the scetch and texture it.
You can buy a Graphic pad and draw then digitaly.
2. make a map editor
Yes this can be done. There are a few games in the source code database that hve level editors. But i doubt it would be easier to make levels. Because you have to learn to make them in the first place in order to learn to make the level editor.
I would read about how this is being done online more.
Google is your friend.