Abing, how do you evolve your Pokemon in Pokemon Go? Also, Great Balls are proving to be not so great. They don't seem to snap up high-level Pokemon or even very low-level Pokemon. Talk about frustrating.
When you tap on a Pokemon, there's an option to "Evolve" right below "Power Up." You'll be using candies to evolve. There's a required number of candies (different for each pokemon, and each evolution). Like, for Pidgeys, you only need 12 to evolve to Pidgeotto. You get the candies from catching pokemon (3 for each catch) and from transferring your pokemon to the professor (1 candy per pokemon). Pidgeot (I think) requires 25 or 50 candies to evolve.
Magikarp needs 400. xD xD Gyarados is freaking special.
It might help, when you're catching pokemon (with either great ball or just pokeball) to spin/shake your pokeball before throwing it (you get curveball exp bonus +10). Also, use try to get great throw as much as possible (+50xp) and high chances of catching high-level pokemon.