A quote from the pokepedia entry on this seems to explain it rather well
"There is a glitch[3] (called the "Old Man trick") in Red, Green(Japan),and Blue that allows a player to battle nearly any Pokemon they wish, depending on the characters of their name. The player should start by viewing the Old Man's Pokemon catching tutorial in Viridian City. Immediately afterward, the player should Fly to Cinnabar Islandand Surf on the east coast (the half-land half-sea tiles). They should not, at any point, swim onto a fully-water tile. Depending on the characters in the player's name, they may eventually encounter MISSINGNO.."
There is no guarantee that you will ever actualy find it but this will increase your chances quite a bit.