In the Pokémon TV series, Pikachu is the first Pokémon Ash sees after waking up on his first journey day.
At the start of the original series, Ash oversleeps and arrives late at Professor Oak’s laboratory. The usual starter Pokémon have already been taken, so Oak gives him a stubborn Pikachu instead. Pikachu initially refuses to stay inside its Poké Ball and does not immediately trust Ash.
Their relationship changes after Ash protects Pikachu from a flock of wild Spearow. Pikachu then chooses to ride on Ash’s shoulder, beginning the partnership that defines the television series. Although Ash later catches and trains many Pokémon, Pikachu remains his constant companion.
Ho-Oh is also seen during the first episode, but it appears later while Ash is traveling. That makes Pikachu the answer to the question about the first Pokémon Ash encounters after waking and beginning his journey.