Pikachu, your rival's Eevee and the Boulder Badge
Start Yellow with Pikachu, build a team that can beat Brock and understand how the rival's Eevee will develop.
International Game Boy Pokémon Yellow Version: Special Pikachu Edition, released in North America in 1999 and Europe in 2000. It plays in colour on Super Game Boy and Game Boy Color. This guide covers the original cartridge, not Let's Go, Pikachu! or the Virtual Console notes.
Pikachu is your only first partner
Professor Oak's lab offers a single Poké Ball, and your rival grabs it first, so Oak gives you a wild Pikachu instead. It follows you on the map and does not ride in a Poké Ball. Talk to it often: its reactions show how it feels about you, and a happy Pikachu matters for at least one later gift Pokémon. It refuses a Thunder Stone, so plan on keeping a Pikachu rather than a Raichu.
Your rival's Eevee and the first battles
Blue takes an Eevee and challenges you in the lab, then again on Route 22. What happens in those early battles decides which evolution his Eevee eventually becomes, so his final team finishes with Jolteon, Flareon or Vaporeon. Win or lose, the story continues; losing an early rival battle never ends the game.
Prepare for Brock
Electric attacks have no effect on Brock's Geodude and Onix, which are part Ground type. Catch a second Pokémon before Pewter City: Mankey on Route 22 learns Low Kick early, Nidoran♂ gains Double Kick, and a Caterpie trained into Butterfree learns Confusion at level 10. Bring a few Potions, because Onix can use Bide to return damage.
If a battle is going badly, check whether the problem is a type immunity before training longer: Pikachu cannot touch Ground types with Electric moves at any level.
Picture details
Game artwork belongs to its respective rights holders.
References
Covered edition: International Game Boy Pokémon Yellow Version: Special Pikachu Edition, released in North America in 1999 and Europe in 2000. It plays in colour on Super Game Boy and Game Boy Color. This wiki covers the original cartridge, not Let's Go, Pikachu! or the Virtual Console notes.