Its true that you can battle/catch missingno with gameshark.
but now,you can catch missingno. with only your gameboy!!!

first,go to viridian city.then,talk to the old man that will teach you how to catch a pokemon.allow him to teach you then go to cinnabar island.surf at thw water beside the volcano(badge)gym.put rarecandy/nugget or anything that you want to duplicate in the sixth place in your bag.you will encounter starmie/marowak level 180,electrode/snorlax level 146,MISSINGNO LVL 80 or lvl 0.WAIT!!!DO NOT CATCH THAT GLITCH POKEMON!!!
if you catch missingno,it will abuse your game.you CANT select the "NEW GAME" in your pokemon red/blue.missingno is like a "HACKED POKEMON" because you cant understand his face.it will also BROKE your "HALL OF FAME",but DONT WORRY.you STILL have a record of hall of fame but it is only broken.i hope i helped!!!