The game begins in Pallet Town. Before going downstairs, interact with your computer in the top-left corner to withdraw a Potion.
Walk downstairs and speak with your mother. It’s time to get your very first Pokemon from Professor Oak. If you go to Professor Oak’s Lab, you’ll see he isn’t there. Go north into Route 1 to find the Professor.
After a little conversation, you will receive a Pikachu and your Rival will get an Eevee.
Your Rival will challenge you to a battle as you walk out. This battle is a bit of a big deal in Yellow. You may want to save before battling so you can restart if you lose. Your Rival’s Eevee will evolve to Jolteon, Vaporeon, or Flareon depending on two battles, this being one of them:
If you beat your Rival at Prof. Oak’s Lab and at Route 22, Eevee will evolve into Jolteon.
If you beat your Rival at Prof. Oak’s Lab and lose or pass up the battle at Route 22, Eevee will evolve into Flareon.
If you lose to your Rival at Prof. Oak’s Lab, Eevee will evolve into Vaporeon.
Attack with everything you have. You will only have two attacks, an offensive and a status attack, so this isn’t a very tough battle. If your Pokemon’s HP gets too low, use the Potion you withdrew from the PC to heal them.
After the battle, leave the lab and go north to Route 1.