Understanding the odds
As far as finding shiny Pokemon goes, the odds are against you. You have a 1 in 8,192 chance of finding one. In games like Diamond and Pearl, there are ways to increase your odds, but for the most part, those are the statistics you're dealing with. Keep in mind that you can find shiny Pokemon anywhere, even though there's no surefire way to get one aside from a direct trade. This means you can catch one by breeding or by catching it in the wild.
Try chaining
Chaining can only be done in the fourth generation of Pokemon titles and is another way to improve your chances of finding a shiny Pokemon. These titles include Pokemon Pearl, Diamond and Platinum. Chaining involves setting yourself up to encounter the same Pokemon over and over again, upping your odds of finding a shiny and allowing you to decide which kind of shiny Pokemon you will find.
The idea works around building a chain using the Pokeradar, an in-game device that lets you see Pokemon in the grass that you wouldn't be able to see otherwise. Using repels and a lot of Pokeballs, you should eventually find a shiny Pokemon if you are able to create a large enough chain and maintain it long enough.
Find someone willing to trade
The only sure way to get one is to find someone who has a shiny Pokemon already. You can even search the Internet for someone willing to trade. There are also Pokemon conventions and tournaments where you can meet with other Pokemon fanatics who may be willing to make a swap with you. Keep your eyes peeled and you'll be able to acquire your shiny Pokemon soon enough.