If you've ever played any of the main Pokémon games or watched the animé, then you should already be able to tell what the plot is. To put it simply, you're a Pokémon Trainer. You start out in one of the four main regions based on the handheld games, obtain a Pokémon from the corresponding Professor, and set out on your journey.
Only this time, you aren't limited to getting eight gym badges, stopping Team Whatever, and becoming the Pokémon League Champion. I mean, sure, you can follow that formula if you really want to... But what's stopping you from joining Team Random and quickly ascending to the top of that crime syndicate's boss ladder? Or what's that, you want to BE a gym leader and crush the dreams of all the aspiring trainers that challenge you? Fine by me. Feel like traversing through all the regions and catching every single Pokémon? That will take awhile, but go for it. I will say this though, for the sake of starting out, you're still a kid. I won't go that harsh on you and make you ten, but instead you're going to be fifteen. Somewhere along the way there will be a way to choose your gender, but for now, everyone you meet will assume you're a boy. You'll have to prove them wrong by branching off the story.
This is an open-ended Pokémon adventure. Do what you want, as long as you do it in a grammatically-correct way, that is. Also, be realistic. No, your Charmander isn't going to evolve straight into a Charizard after battling one Pidgey. And no, that Charizard isn't going to fly you directly to Mewtwo only to have it beg you to let it be your Pokémon before promptly catching itself in one of your Poké Balls. I mean, come on... Sure, you can certainly catch Mewtwo eventually, but like I said, be realistic and have your story make sense.
With all of that said, there's only one question remaining: Which region do you want to start your adventure in?