Clicking the right mouse button will instead replace the foreground color on your canvas with the background color you have selected. That is, if you click the left mouse button. The eraser does exactly as you'd expect it to: erases pixels you've already placed. For pixel art, you should always use a pencil. Brushes, meanwhile, may fade in/out at the beginning and end of a stroke. The pencil does not have anti-aliasing, so it always produces a hard edge. Typically, the difference between the pencil tool and the brush tool in art programs is anti-aliasing. You can also select whether you'd like to use a circular pencil or a square one.
In Aseprite, you can scale the pencil's size up to 64px. This is your main drawing tool, which by default, places one pixel.
