You find pokemon in tall grass. Once you start off the game, go to the tall grass once you go out of your home. Professor Oak will approach you and take you to his lab, where you can select a starter pokemon. However, in Pokemon Yellow you can only obtain Pikachu, while your rival obtains Eevee. You catch pokemon with poke-balls. You can buy them at a poke-mart in any town in the game. However, you'd have to deliver Oak's parcel from the poke-mart in Viridian city. Go to the poke-mart and talk to the shopkeeper, he will hand you the parcel and send you to deliver it to him. Once you deliver it to him, he gives you a pokedex. Then you're free to go. Once you arrive at Viridian city once again, you can buy pokeballs from the poke-mart and catch any pokemon you'd like. You're welcome
-Mark