TL;DR Summary
Oceana Pier - Items - X Accuracy (Hidden Item), X Defend (Hidden Item), Sweet Heart (Hidden Item)
Gearen City - Choose your starter, battle Amaria, battle Ren
Feel free to explore around Oceana Pier to hear about how devo everyone is the S.S. Oceana sank. You'll be coming back to this place over and over again throughout the storyline, so don't get too hung up on doing so. There's also a few items here as well:
Amaria will walk out afterwards. Jenner will speak to you, and ask you to go fetch Melia, his daughter, who is training to become a Gym Leader. Before he can finish, Ren comes in and interrupts saying he finished his research. After he leaves, Jenner will finish telling you about Melia, who is in Goldenwood Forest, which can be accessed by the North Railnet. For your troubles, he'll give you Running Shoes, Pokedex and a CyberNav. Before you are allowed to leave the Building, Ren will be a dick and block your exit. He'll inform you that Melia is actually in Gearen Sewers at the moment and not at Goldenwood Forest. Apparently being helpful will cost you a battle though - He'll have the starter starter that is weak to yours/Froakie:
Oceana Pier - Items - X Accuracy (Hidden Item), X Defend (Hidden Item), Sweet Heart (Hidden Item)
Gearen City - Choose your starter, battle Amaria, battle Ren
Feel free to explore around Oceana Pier to hear about how devo everyone is the S.S. Oceana sank. You'll be coming back to this place over and over again throughout the storyline, so don't get too hung up on doing so. There's also a few items here as well:
Head up and exit into the gate, and go through into Gearen City. Most of the paths are blocked off because "you ain't tough enough", etc. Follow the path into the big building, where you'll be greeted by Amaria. Follow her to the desk, and she'll lead you through to Prof. Jenner's Laboratory. Before you can step through the door though, you'll bump into ?Melia, who will be a major character later on. Follow into Jenner's room, who will give you the spiel about choosing carefully, etc.
Once you enter the room, you'll see that it's divided into three sections for each of the types for starter Pokemon. Notice that the Gen 6 starters are not available here, but will be later on in the game. The Oshawott will follow you around the room begging you to choose it, which is kinda hilarious. NB: You can get Fennekin, Froakie, Mudkip, Oshawott later on as one-off event Pokemon. You can also catch a Chesnaught, which I believe means you can breed a Chespin as well if you wanted to.
Anyway, pick your starter and head on back outside. Amaria will immediately challenge you to a battle, which will be the starter with the advantage against yours at Lv 5 (i.e. if you chose Turtwig, it'll be Fennekin, etc.). After you defeat her, she'll hand you some Pokeballs:
Defeat him, and he'll tell you to explore the city, which you should probably do before you venture into the sewers.