(Alright. So, I got everything I need time to start. Warning: this is my first CYOA so please feel free to creatively criticize me, as well as give me advice.)
Your name is Lance, an 18-year-old with a dream of becoming a Pokémon master. You were the oldest person in Pallet Town without a Pokémon seeing as how Red, Blue and even a teen named Ash beat you to getting the last ones. Since they weren't around, you finally have a chance to get a Pokémon.
Fully rested, you got up from your bed and got dressed. You wore a black cap with a pokéball logo on the front on top of your black hair. You fixed your round glasses, so your silver eyes could see. You wore a white t-shirt, black jeans, gray sneakers and a black backpack. Walking down the stairs, you see your mother in the kitchen, cooking up an egg sandwich for you.
Your mom looks up and sees you with a small smile. She wore a black apron, pink t-shirt, navy blue jeans and black sandals. Her black hair reached to her shoulders and her eyes were pink to match her loving and caring nature.
"Morning Lance," she says happily. "Go ahead and sit down for your breakfast."
When she looks away, you admire her figure, specifically her huge breasts and her round ass. You always found your mother hot, which made you feel a little bad since she took care of you your whole life. That didn't change your mindset though, seeing that you would love the idea of you getting in her pants.
"I'd love to mom, but I gotta go before Professor Oak runs out of Pokémon," you quickly tell her.
"Oh yes, go on. But hurry back, before your sandwich gets cold." You nod to her as you quickly run out of the house and heads towards the lab.
Later, at the lab...
You enter the lab to see scientists running left and right frantically, seemingly worried about something. One of the scientists notice you and pointed to the back of the lab. Heading there, you see a man with blue eyes and brown hair. He wore a lab coat, red collared shirt, tan pants and brown shoes. He gave you a small smile, which didn't hide his worried look.
"Oh, good morning Lance. Sorry about the mess, something... strange... has happened," the old man said.
'"No problem, Professor Oak," you tell the Pokémon Professor.
"Now, shall we get to the matter at hand. I understand that you're getting your first Pokémon. If you look at the table..." He points towards the table, which held three Pokeballs. You notice that a fourth one was on the floor. "Those three Pokeballs have a Charmander, fire-type, a Bulbassaur, plant-type, and a Squirtle, water-type. Please make your choice and select a Pokémon."
You look at the three Pokeballs, as well as the fourth on the floor, and made your final decision...
Choose your Pokémon (This doesn't affect the story, also if you want you can suggest what the Mystery Pokémon could be in. Depending on what the Mystery Pokémon is, it might be the only one that can affect the story):
www.strawpoll.me/12142655 
