How do I draw 16 / 8 bit?

Hey! I just got Aseprite and I want to make pixel art, but how do I make it look like mario world looking graphics and not a drawing with AntiAlasting on?

That is kind of a loaded question but for starters, set your brush size to 1 pixel and zoom in further.

If you already have a reference for what you want your artwork to look like, one of the best ways to learn is to examine and reference as you create. The sprites for that game (and basically any from that era) are widely available to view and play with, just google the game and character followed by “sprite”- check the resolution of this artwork, the number of colors used and how they achieved that look within those constraints. Try working within those same or similar constraints and it should be easier to progress.