To get the glitch pokemon missingno you must preform some tasks. First Talk to the old man with the coffie in Viridian City. Say Yes To Him he will try to catch a Pokemon. (Also please take note this trick won't work in Yellow)

Fly/Teleport To Cinnabar Island

Surf up and down like this

Not only may you get a Missingno You May get a M or a way too powerful pokemon