The best eraser to use when learning engineering graphics is a white vinyl eraser.  These are available in block form or in a convenient pen package.  Some (but not very many) mechanical pencils come with a quality vinyl eraser.  Be sure to check the quality of the eraser.  You will probably need to purchase an eraser separate from your mechanical pencil.

Staedtler Brand White Vinyl Eraser (Staedtler,

When drafting was done manually, electric erasers were employed to speed the process.  There is little reason to purchase one of these as a student.

Electric Eraser (