After you venture through Route 17, you manage to get into Po Town to fight Team Skull.
Explore Po Town
Go to the bottom-right of the area to get a Max Repel.
Then go to the left to find a hole in the wall. Check on it to go through. Then go east toward the barricade and you have to battle a Team Skull Grunt who has a level 36 Spinarak. You get 1152 Pokédollars for winning.
After that, you have to battle the other Team Skull Grunt. He has a level 36 Trubbish. You get 1152 Pokédollars for winning.
You can go into the Pokémon Center, but they charge you 10 Pokédollars to heal up your pokémon.
Go north and you reach another barricade. If you go up to the barricade, you battle a Team Skull Grunt who has a level 37 Drowzee. You get 1184 Pokédollars for winning.
If you go to the right and go through the hole there, you have to battle two Team Skull Grunts in a double battle. They have a level 37 Golbat and a level 37 Raticate. You get 2368 Pokédollars for winning.
Go north from them to find a Full Restore.
This is a dead-end, so go back through the hole between the Grunts. Then go west and go through the hole in the north wall.
Go north and you can battle the Team Skull Grunt on the truck. He has a level 37 Ekans and a level 37 Salandit. You get 1184 Pokédollars for winning.
North of the truck, there is another barricade. Go to the left, and walk on the boxes to go through the hole in the fence there.
Near the fence, you can battle a Team Skull Grunt who has a level 37 Fomantis and a level 37 Mareanie. You get 1184 Pokédollars for winning.
Go all the way to the right, then check the south wall where the arrow is pointing at it. After going through the wall, you find a Smoke Ball. Go back through the hole.
Go north and open the doors of the mansion to enter Shady House.