Your journey starts in your bedroom. Head downstairs to meet your Mom. After a brief chat, head outside. Your rival, Barry, will immediately crash into you. Follow him north to Lake Verity. Here, you'll encounter Professor Rowan and his assistant, Dawn/Lucas. After a brief encounter with a wild Starly, Professor Rowan will entrust you and Barry with your first Pokémon. Choose wisely between Turtwig, Chimchar, and Piplup!
Once you have your starter, head back to Twinleaf Town. Your Mom will give you the Running Shoes, allowing you to move faster by holding the B button. Barry's Mom will give you a Parcel to deliver to Barry. Head north again through Route 201 to Sandgem Town.