Viridian City is your first major stop. Upon entering, you'll notice a large building in the center – the Pokémon Gym. However, it's currently closed. Your immediate goal is to deliver Oak's Parcel.
Head to the Pokémon Mart, the building with the red roof. Inside, talk to the clerk. He'll take Oak's Parcel and, in return, give you a Poké Ball and a Potion. He'll also mention that Professor Oak wants to see you. Now, head back to Pallet Town to speak with Professor Oak.
While Pikachu is a fragile early-game attacker in Pokemon Unite, the mascot serves as your sturdy, permanent companion when first navigating the tall grass of Viridian Forest.