In Sandgem Town, visit Professor Rowan's lab. He'll explain the Pokédex and send you on your way. His assistant will give you a Town Map. Heal your Pokémon at the Pokémon Center and stock up on supplies at the Poké Mart if needed. Head north onto Route 202.
On Route 202, you'll have your first trainer battles. These are good opportunities to gain experience for your starter. Continue north to Jubilife City. Upon entering, you'll be greeted by a man who will give you a Pokétch (Pokémon Watch) after you find three clowns around the city and answer their questions correctly. The clowns are located near the Pokémon Center, in front of the TV Station, and near the Global Trade Station (GTS). Once you have the Pokétch, explore Jubilife City. You can visit the Trainer's School for a Potion and a Super Potion. When ready, head east onto Route 203.