Design, graphics isn't my strongest point
Still not sure which you mean
level design by itself is just architecture and mechanics
graphics are aesthetics
but yeah, with both my answer would be: look at games, start simple. 8bit level design and graphics to get into it
Making your own map editor can be a chore... you CAN use existing ones like "Tiled
but you can also just start simple...
here is what my first map-editor looked liked (Top down in this example)
its gets better over time
later you want to place enemies and stuff, have many layers...
but when you start, just keep it simple