However, if performing a glitch do not count as cheating, then Missingno. can be found fairly commonly on the line of ocean tiles along the east coast of Cinnabar Town in Red, Blue and Yellow. Its specific chance of appearing depends on your trainer name, and the rest lies in the coding of the game. It would be too hard to go any further than this since the result differs too much (this being a glitch and not something intentionally put in the game and all).
Can you capture it?
Is there a way to prevent Missingno. from appearing ever?
If you don't do this Old Man glitch, it will never appear. And doing the Old Man glitch unintentionally is basically impossible, so don't worry. The glitch only exists in Red, Blue and Yellow, so if you are playing a different game, Missingno. doesn't even exist.