When you first start your game, you will have 1 potion in your pc that's in your room. Get it out. Continue playing the game untill you get Pikachu but haven't battled Gary's Eevee yet. Use the potion on Pikachu 5 times (even though it won't have any effect). Fight Gary and WIN. After the battle, when your Pikachu comes out of the Pokeball. He will like you.