How do you get the red garados in Pokemon Diamond?

Your thinking of Pokemon Silver/Gold/Crystal, which has a Shiny Gyarados, which is part of the Plot, which you will have to fight at one point. Same for Soul Silver and Heart Gold, which also features a Red Gyarados.

In Diamond/Pearl/Platinum, the only way you will encounter a Red Gyarados is in the wild. The chances are 1/8192 in finding a Shiny Pokemon. If you breed a Shiny Gyarados or Magikarp, and it has an egg, the egg has a 1/69 chance of being shiny.