You begin in your house in Twinleaf Town. Head downstairs to meet your mom. After a short conversation, head outside. Your rival, Barry, will immediately drag you to Lake Verity. Here, you'll encounter Professor Rowan and his assistant (Dawn/Lucas). After a brief cutscene, you'll be attacked by wild Starly. Professor Rowan will allow you to choose one of his Pokémon to defend yourselves: Turtwig, Chimchar, or Piplup. Make your choice, defeat the Starly, and then return to Twinleaf Town. Your mom will give you a pair of Running Shoes. Now, head north onto Route 201.
On Route 201, you'll encounter Professor Rowan again, who will officially entrust you with your chosen starter Pokémon and give you a Pokédex. His assistant will give you 5 Poké Balls. Continue east to Sandgem Town.