Just be lucky. Keep fishing :P
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The Gold Magikarp is a shiny Magikarp. It evolves into a red Gyarados. There is a 1 in 8912 chance that the Pokemon you encounter is shiny. This can be reduced to 1 in 2048 chance by using the Masuda method. This is basically when you breed a Magikarp with a foreign Ditto. The egg has a 1 in 2048 chance of being shiny.
Your best bet would be to use the "all Pokemon shiny" code for your game then go fishing in the Resort Area for the level 100 Magikarp that can be caught there. In Platinum, that is.
It depends on luck. when catching a magikarp, you have a 1/8000 chance of shiny.
There is no way to get a Shiny Magikarp 100% of the time.
A golden Margikarp is a shiny margikarp, and as with other shiny Pokemon, there is only a 1/1892 chance of getting a Shiny(golden) magikarp. Breeding a Shiny Gyarados or a Shiny margikarp with another shiny Pokemon THAT CAN BREED WITH GYARADOS/MARGIKARP may have a higher chance of breeding a shiny magikarp. You can also trade the shiny gyarados from Pokemon Gold, Silver, and Crystal version.