You start as a Pokémon trainer who has been called by Prof. Elm Jr. After getting to his lab, you are allowed to choose a starter pokemon out of 10 (I'll post the list of starters later).
Remember, starters are first come first serve. The player whose SU is accepted first gets the most choices (10), and the 10th player would have no option but to take the starter left over (just like Ash, who had no choice but to take the Pikachu).