You can't.
Magikarp will evolve into Gyarados.
Raichu cannot be found you must evolve a pikachu to get one.
Magikarp evolve at level 20.
Magikarp can evolve into Gyarados, starting at level 20.
Magikarp evolve at level 20.
A Magikarp can evolve into a red Gyarados. The Magikarp needs to be a shiny Magikarp in order for it to evolve into a red Gyarados. This is due to the red Gyarados simply being a shiny Gyarados.
Get the Exp. Share and let your Magikarp hold it. When you battle, use a strong Pokemon and Magikarp will get some Exp. That's how I got my Magikarp to Evolve.
A Magikarp should evolve at about lvl 40 on Pokemon diamond
Magikarp evolve at level 20.
Magikarp evolve at level 20.
Magikarp evolves at level 20.