Just like with every other Pokemon game, the player is allowed to select their own starter Pokemon. Pokemon Go only has Generation 1 Pokemon right now meaning the starters are Charmander, Squirtle and Bulbasaur. There is a hidden starter Pokemon as well however, if players are willing to ‘run away’ from the main starters they will eventually find a Pikachu who will be their starter Pokemon instead.
In the main games, the starter Pokemon was a center point member of the team but this is a little different in Pokemon Go; there is no level system that puts all Pokemon in the same area of usefulness, the game uses a Combat Points system instead.
Combat Points raise when a Pokemon is evolved and when the player spends some in-game resources to level them up. There is a limit on how high this can be raised and the lower the starting CP, the ‘weaker’ the Pokemon will be.
The starter Pokemon will have very low CP value overall which does not make them ideal fighters; to level them up, the player needs to catch multiple Pokemon of the same species and transfer them to the Professor in exchange for candy. Once enough candy has been collected, the player can evolve it to the second stage. To evolve it to the final stage, the player needs to obtain more candy by transferring Pokemon of the same species.
It takes a lot of work to evolve a starter Pokemon as they are difficult to find in the wild and hatching them from eggs is tedious. Charmander seems to be the rarest one of the group but that doesn’t mean the others are commonly found.
Does the starter Pokemon matter in Pokemon Go? From a gameplay point of view, it doesn’t matter too much and many players transfer their starter to evolve a stronger Pokemon of the same species.