Until now I am following the nerdy nights tutorial and the adaptation for CA65. In those tutorials the authors give you a chr file for the graphics. To put it simple, in the CHR file you will put all the graphics that you will use in tiles.
Here is an example:
I tried to create a new CHR file from scratch BUT as it is not completly filled it creates kind of a problem with memory, so I took the Mario chr file and started to edit it. I created a simple character in 5 minutes (my daughters want to be the ones that creates the definitive character for the game, so this is something I will use for testing and developing).
Editing this will also take a HUGHE ammount of time, but in this hobby you enjoy the trip.