Missingno. is a glitch Pokemon. If caught, it will cause some minor glitches in the graphics. It is pretty hard to catch, seeing at it exists only under certain conditions. And here they are:
1. Make sure your character's name is ASH.
(Added note: There are other names, but ASH is the easiest to remember for me.)
2. Go to Viridian City and talk to the old man near the top. He will ask if you're in a hurry. Say "no."
3. After you're done talking to the old man, fly to either Cinnabar Island of fuschia City right away. Don't do anything else until you're there.
4. If u flew to Cinnabar Island, Surf to the east coast. If u flew to fuschia City, Surf to the Seafoam Islands.
5. Once u get to the coast of Cinnabar, Surf along the coast. of if at the Seafoam Islands, Surf around the coast of the islands. Either way, u will eventually encounter the elusive Missingno.
Hope this helped!
- Plasma Grunt Michael