At the beginning of the game, you have the choice of three Pokémon to start your adventure. You receive any of the following Pokemon from Professor Elm's Lab at the beginning of the game. You receive them at level 5.
After you receive all 16 badges and beat Red, you can visit Professor Oak to receive any of the following starters. These are the starters of Kanto. You receive them at level 5.
After you receive your Kanto starter, you can get a Hoenn starter by visiting Copycat in her pink house in Saffron. When you talk to her, she will tell you of her lost doll. To get it back, go to the Pokemon Fan Club in Vermillion City and talk to the man at the table. He will return the doll. Upon leaving, you will meet Steven Stone of Hoenn. He will start the Latias/ Latios in-game event. After you return the doll, you can visit him at Silph Co. and he will give you a choice of any of the Hoenn starters below. You receive them at level 5.