After you kick Team Flare out of the Poké Ball Factory with the help of your neighbor, go east to Route 15.
Moves to Use
If you want to collect everything in this route, bring pokémon that know Surf, Rock Smash, and Strength. There are items that will still be inaccessible, but you can get them later when you receive the HM for Waterfall.
Battle and Catch
Go east onto the leaves, then walk south past the tall grass. On the left side, there is a pile of leaves that is actually Pokémon Ranger Silas, who has a level 39 Quagsire and a level 39 Beartic.
Continue south past Silas and walk through the pile of leaves at the end of the path to find a Revive.
Now go north, then east. Walk through the piles of leaves to find a Net Ball.
South of there, you can battle Hex Maniac Luna. She has a level 40 Chimecho.
Go north from the Hex Maniac and walk to the right, jumping over the ledge. There is a Leppa Berry tree here. Use your Dowsing Machine to find a hidden Antidote to the left of the tree.
Now go south from the tree and hop down the ledge. Pokémon Ranger Dean will pop out of the leaves here. He has a level 37 Watchog, a level 38 Swoobat, and a level 39 Seadra.
East of that ranger, Pokémon Ranger Keith is hiding in the leaves. He has a level 41 Gabite.
Use Surf in the water to the right of the Pokémon Ranger and then surf north. Get off on the right side and walk through the pile of leaves to get a PP Up. Use Surf to go across to the western side, then use the Dowsing Machine to find a hidden Tiny Mushroom.
Surf south and get out of the water near the Pokémon Ranger from earlier. Go south through the tall grass. There is a pile of leaves west of the bridge where Pokémon Ranger Pedro is hiding out. He has a level 41 Vileplume.
Walk through the pile of leaves west of Pedro to find a Dire Hit.
Now go east and cross the bridge. Go south into the tall grass. There is a narrow path to the south that goes along the river. Walk along the path to reach some stepping stones in the water. Hop across to find a Full Heal. Then jump across again and walk along the path until you are near the bridge.
Northeast of the bridge, in the broken building, there is a cracked wall that a pokémon can break with Rock Smash. Break the wall, then go north to get a Protein.
The eastern wall of this ruined building can also be broken with Rock Smash. However, you can't get any items past that wall because you don't have the HM for Waterfall yet. You'll have to come back here later if you want the item.
In front of the ruined house, the Mysterious Sisters Rune and Rime will challenge you to a double battle. They have a level 40 Floette and a level 40 Drifblim.
Go south of the sisters and use Surf to get into the water. Go south in the water and hop off at some grass east of a few rocks in the water. Use your Dowsing Machine to find a Pretty Wing on the south wall. This item does not do anything, but it can be sold for 100 Pokédollars. However, there might be someone in Kalos who will pay more for it.
You can't go any farther south in the water, so go north and get out of the water near the Mysterious Sisters in front of the ruined house.
East of the sisters, there is a bridge. When you go east over the bridge, you receive a Holo Clip from Lysandre, who speaks briefly about Mega Evolution.
East of the bridge, there is a Trainer Tips sign. Northeast of that sign, there is a cracked wall that you can break with Rock Smash. Behind the wall, you find a Macho Brace. A pokémon who holds it will earn double EVs. It stacks with Pokérus, so a pokémon that has pokérus or had it in the past will earn four times the EVs if it wears a Macho Brace.
Go back outside and walk south. You find Fairy Tale Girl Mahalyn running around. She has a level 37 Dedenne and a level 39 Aromatisse.
East of the Fairy Tale Girl, there is a gatehouse that leads to Dendemille Town. However, there is still more exploration that you can do here.
To the south of the Fairy Tale Girl, you will reach Route 16: Mélancolie Path. Use Strength to push the boulder out of the way. Route 16 is optional, but you can get a bunch of items there, including the Super Rod, as well as plenty of experience battling trainers.
North of the gate to Dendemille Town, you can go into the ruined house, but first you must battle Hex Maniac Carrie. She has a level 38 Litwick and a level 38 Duosion.
In the house, go up the stairs on the left and turn on your Dowsing Machine to find a hidden X Defense upstairs.
Go downstairs and go to the east side of the house. There are some stairs leading down into the Lost Hotel.
Where to Go Next
You can optionally go south from Route 15 to explore Route 16: Mélancolie Path, which leads to Lumiose City. You can get the Super Rod in Route 16. Be sure to bring a pokémon that knows Strength.
Another optional destination is the Lost Hotel. The hotel is split into a northern half and a southern half. The entrance to the northern half is in Route 15, and the entrance to the southern half is in Route 16.
When you're ready, go east to Dendemille Town.