Imagine Pokemon are real. They don't suddenly appear and you're not teleported to the Pokemon world, but they've been a part of our lives forever and our world has the Pokemon infrastructure you'd expect.
Think of the areas you regularly played at when you were a kid. Then look here to see what Pokemon would likely be in those areas, realizing that a tiny stream wouldn't have Gyarados or Feraligatr in it despite those being listed under streams and rivers (http://www.theworldofpokemon.com/habitats1.html)
If you want you can look at this difficulty list to further narrow down your choice (http://www.theworldofpokemon.com/difficulty.html) otherwise just feel free to pick whatever you like most from the low level Pokemon that'd be in your area
What Pokemon do you go for? Do you think (or hope) that your life would pan out differently from how it did? Do you think you'd fulfill your childhood dream of being a vet/firefighter/archaeologist, or do you think you'd end up as a member of a criminal organization? Would you be content being an office worker who goes out to the woods or the mountains on the weekend to train their Pokemon, or would you want to challenge gyms?

When I was a kid I lived on the outskirts of a small town in a cluster of 2 dozen houses surrounded by cornfields and thickets. Fields and meadows would probably be the closest equivalent even though those imply less human activity. I could see myself catching a Shinx, Sentret, or maybe Patrat as my first Pokemon. I really wanted to be a police officer as a kid, so if I actually saw that through then Shinx and Patrat would be the best choices since thunder fang, bite, roar, thunder wave, and hypnosis would all be very useful moves
But honestly I probably wouldn't amount to anything and in my desperation I'd join a criminal organization (as long as they care for Pokemon). A Shinx would still be fitting, but I could also see myself befriending a Murkrow over time by giving it random shiny things.