When you first load up the game, before you're shown the opening cutscene, you'll first be given the choice of which language to play Pokémon Omega Ruby or Alpha Sapphire in. Whichever language you choose will be the language the entire game is in and it is unable to be changed unless you delete your save file, so choose wisely!
Once that's out of the way, you'll be greeted by an all-too-familiar screen. That's right, it's Professor BIRCH, and... no, don't adjust your television set; despite the fact it looks like you're playing the original Ruby and Sapphire, after Professor Birch says a few lines, the screen's angle will change, and then it will be shown that it is just something that you, the player, are playing.
Needless to say, though, the Professor keeps talking, except now you're given a more scenic, 3D view of the surroundings, seeing Pokémon in your natural habitat.
After some more explanation, Professor Birch has a very important question for you: are you a boy or a girl? Keep in mind that there are no different skin tones or hair styles, unlike Pokémon X & Y, but instead the player characters are that of Brendan (the boy) and May (the girl). Whichever choice you select, your rival throughout the game will be of the opposite gender. He'll also ask you for your name.
Now that that's settled, once the truck stops, hop outside of the back of the moving truck. You'll be greeted by your mother, and she leads you into your house. She'll tell you to head upstairs to set your clock. Go ahead and do so, though you'll just set the clock to match whatever time it is on your Nintendo 3DS.
Once that's out of the way, head back downstairs and then your mom will call you over, saying that your dad may be shown on TV, but it turns out the two of you just barely missed seeing him. Your mom then tells you to meet one of your dad's friends, Professor Birch, who lives right next door.
Head on over to the house next door and you'll be greeted by Professor Birch's wife, who mentions that she has a son or daughter (based on your gender) about the same age as you waiting upstairs, and he or she would like to meet you.
Upstairs, you'll trigger a little cutscene with either Brendan (if you're playing as a girl) or May (if you're playing as a boy). After some talking, he or she will head off, saying he or she needs to help his or her dad catch wild Pokémon.
Now you want to head north, onto Route 101, in order to find the professor. However, a nearby kid will mention the screams heard up ahead and tell you that someone needs to help. That someone is indeed you!
On Route 101, you'll find Professor Birch being chased by a Poochyena, and he begs for your help! He instructs you to grab a Pokémon from his bag to ward off the Poochyena. Go over to the bag on the ground and press A by it (you can save beforehand should you want to reset for whatever reason).
You'll now be able to choose between the Grass-type Treecko, the Fire-type Torchic, and the Water-type Mudkip. Mudkip is most likely the best choice of the three, especially once it evolves, as it has excellent coverage against most of the Gyms in the game. Torchic also becomes quite strong upon evolving and has alright coverage, while Treecko is definitely the most challenging of the three to use, as it has much less super effective coverage against the Gym Leaders in the Hoenn region.
Whichever of the three you decide to use, you'll have to use that Pokémon (it starts at level 5) to fight against a wild level 3 Poochyena. It should be an easy fight; just use your starter Pokémon's first attack and you should have no difficulty in knocking out the Poochyena.
After winning the fight, Professor Birch will take you back to his lab, where he'll give you the Pokémon you used to fight the Poochyena. You'll also be given an opportunity to nickname that Pokémon.
Whether you nickname your Pokémon or not, Professor Birch will tell you that his son or daughter is on Route 103 surveying Pokémon and suggests that you go to see him or her. He also tells you that you can use the AreaNav on your PokéNav Plus to find the way there.
Now it's time for you to make your trek north to Route 103, traveling through Route 101 and Oldale Town before you make it there!