You have a 1/8192, or 0.0001220703125 %, chance to catch a shiny in the wild. These are really rare so be sure you catch it! You could also trade for a shiny, but they basically have the same chance as you.
You have a 1/8192 chance of finding a Shiny Pokémon in the wild in Pokémon Black and White.
The Masuda Method is the best way to get a Shiny Pokémon in Black and White as it multiplies the chances of getting a Shiny Pokémon by 6 but even this requires a lot of time and patience. The Masuda Method involves breeding a Pokémon from your game cartridge with a Ditto from a foreign game cartridge. The egg that you receive from the Day Care Man has a 3/4096 chance of being Shiny or about a 1/1365 chance of being Shiny.
For more information about the Masuda Method click here
Source: http://bulbapedia.bulbagarden.net/wiki/Shiny_pokemon
Hope I helped :)
