You have a 1 in 8,192 chance of encountering a shiny Pokemon. So, you have to either run around in the grass and get really lucky, or you can use the Masuda method, which is when you breed two Pokemon from different countries. The likeliness of getting a shiny then becomes 1 in 1,365.3.
Another way you could is:
> In Pokémon Black 2 & White 2, a new item is introduced called the
> Shiny Charm. This item, obtained when you have completed the National
> Pokédex, increases the chances of Shiny Pokémon appearing. As it's a
> Key Item, when you have the item in your bag, then the chances of
> encountering Shiny Pokémon in the wild is increased to 300% of normal,
> making it a 1 in 2,730.6 chance. If, however, you're using the Masuda
> Method to breed, it reduces it from 1 in 1,365.3 down to 1 in 1024.
Also, a way to get free shiny Pokemon is by beating Banjirou in Black Tower/White Tree Hollow and then going to Alder's house. Banjirou will then give you either a shiny Gible, or a shiny Dratini.
Another free shiny Pokemon is a Haxorus. Once you have seen all the Pokemon in the Unova dex, you will get a License. You show the license to the pilot in Mistralton, and he'll take you to Nature Sanctuary. In the heart of the sanctuary, there is a shiny Haxorus.