A lot of this is covered by the tutorials here...
https://www.thenew8bitheroes.com/learn
NesMaker as standard has pre-determined bmp files for all your monsters, screen specific, main tile sets and objects in /GraphicAssets. For a new project they'll all be blank and can be filled with your own tiles but must retain the same sizes and file names for NesMaker to compile your project. Not sure if this is the info you're looking for but I'll go on anyway...
Generally, your players, pickups, projectile etc would all be in the GameObjectTiles.bmp
Then you've got loads of BckCHR which are your main "background" tiles, in basic terms things that aren't going to be moving (platforms, walkable tiles, ladders etc)
Also, theres BckSS which are smaller 128 x 16 screen specific tile sets which can also be used. Sometimes used in Adventure games to have specific things that might only be on that particular screen (like a unique building or something). However, you don't have to use them that way. They can be used just to give more tile variety over multiple screens. It all depends wheather you intend to have a hud and text eliments and/or path tiles to which tileset configuration you use. For example, my game doesn't use the HUD or path tiles so I can use 2x Main (BckCHR) and 2x BckSS which gives a lot of tiles to work with.
I'd recomend planning out what tile sets you're going to use for what. For example, you might want to save some of your monster tilesets for cutscene animations, or seperate monster tilesets for bosses for example, stuff like that. Plan what you need for your game.
NPCs would generally go in your monster tilesets (i think?, thats how im doing it) but I guess they could be in your game objects bmp if you wanted.
Get some pixelart software. Piskel is free but there's others that are paid and probably better. I just use Piskel because it works for me.
Hope this helps. Really depends what kind of game you're making ?
