Pokémon Omega Ruby and Alpha Sapphire/Littleroot Town

'This is Littleroot Town, your brand-new home. Time to introduce yourself to the neighborhood...and perhaps receive a Pokémon?'

Meet the Neighbors
After finishing the opening cutscene, your mom will ask you to set the clock in your room. Head upstairs and interact with it; it will automatically set the time to match your 3DS's internal clock. Time of day affects several events such as Pokémon evolution and Mirage Spots, so don't fiddle around with your 3DS clock. Next to your clock is a notebook with a few tips on how to open the menu and save your game.

Once that's done, go back downstairs and your mom will suggest you meet your neighbors. In Littleroot, the person of note is Professor Birch, a Pokémon researcher. He and his family live in the house next door to you. Drop on by and meet Mrs. Birch, then go upstairs to greet Brendan or May.

To The Rescue
Professor Birch himself isn't in town right now--he's out studying wild Pokémon on Route 101, the route directly above Littleroot. The game will not let you go into the route until you have spoken to Brendan or May. Once this is done, simply go straight out.

Professor Birch is in a bit of a fix, having been cornered by a wild Poochyena. He's dropped his bag and he begs you to use a Pokémon. Interact with the bag, and the game will offer you a choice of three starters: Treecko (Grass-type), Torchic (Fire-type), and Mudkip (Water-type). Once you've made your selection, battle and defeat the wild Poochyena. A very grateful Professor Birch will offer you the starter you picked, and give you the chance to nickname it. He'll then suggest you meet his child, who is also out in the field, in Route 102 beyond Oldale Town.