In Sandgem Town, Professor Rowan will formally give you your Pokédex and 5 Poké Balls. He'll also introduce you to his assistant, who will teach you how to catch Pokémon. catch a few early Pokémon to start building your team! Good early catches include Starly and Bidoof.
After this, head north onto Route 202. Here, you'll encounter your first few trainers. Battle them to gain experience for your Pokémon. Remember to heal at the Pokémon Center in Sandgem Town if your Pokémon get low on HP. Continue north until you reach Jubilife City.