You're in Undella Town! The Pokemon Center's on the other side of town, but you shouldn't be in desperate need of healing, so let's explore the place before we head there. First take the stairs north of the exit from Reversal Mountain.
Move up the thin path to the left of the truck and grab the hidden Shiny Stone at the end of the path. Head back down the stairs and take the steps leading east. You'll see gaps just before the fence signalling where Undella Town properly begins. Head down the southern gap.
Follow it around until you find the Berry Juice at the end. Then head back to the stairs and through the fence into Undella Town itself. The first building you move past doesn't have anything of importance. Continue to the blue entrance leading down underground.
This leads to the Marine Tube which will take you to Humilau City , the final city before Victory Road and the Pokemon League . Yes, you're getting close! Unfortunately you can't traverse the Marine Tube just yet as there's a crazy old man blocking the road... Let's keep heading east.
Move past the building to the right of the entrance to the Marine Tube as it once again has nothing of importance. Head down the steps onto the sand. Don't head north into the gate there as you'll trigger a Rival Battle (we'll get to that a bit later).
Head up the steps to the Pokemon Center and enter it. Heal your Pokemon if you need to, check out the items being sold at the Poke Mart then talk to the man along the western wall of the Center, near the foot of the left staircase. He'll give you a Prism Scale .
Take it then head over to the foot of the staircase on the other side. There's a shady man dressed in black here who unlocks another Funfest Mission . Once you're done, leave the building.
Head back down onto the sand. Turn on your Dowsing MCHN and use it to find the Pearl hidden in the sand nearby. The little sandy path between the cliff and water heading east will take you to Undella Bay , an optional area for training.
Even if you don't intend to explore it, head there, turn on your Dowsing MCHN and head up the beach. You'll find a hidden item in the sand, a Heart Scale . Keep heading up the beach, following it along even when it becomes very thin. At the very end you'll find a Draco Plate !
If you want to continue exploring Undella Bay , skip to that section. If not, head back into town.
Go west along the beach through town. Just past the last set of deckchairs and umbrellas before the beach becomes thin for a small while, there's another item hidden in the sand. It's another Heart Scale . Grab it then continue along the beach.
The beach sand will become thin then wide again. When it does, follow it as it turns south. There's another Pearl nearby, hidden in the sand. Find it, collect it then head-south west. You'll find some stairs leading up to a rocky area.
That's all there really is to do in Undella Town . If you go back down to the beach and follow it south-east, you'll head into Route 14 , which leads to White Forest if you're playing White 2 and Black City if you're playing Black 2. However you won't be able to enter either location until becoming Champ.
Now head back to the Pokemon Center. Try and head through the gate to the north, taking you to Route 13 . Make sure you're ready for a Rival Battle as Hugh will stop you and promptly challenge you. It's time to get your game on!
If Snivy was your starter
If Tepig was your starter
If Oshawott was your starter
Once he's defeated, go and heal your Pokemon, then head through the gate into Route 13 ! Skip past the Undella Bay and Route 14 sections below if you don't want to explore them. If you do though, skip ahead to the appropriate section and enjoy! They're both optional, but both a bit of fun.