After you make it through Route 17, a mysterious man lets you into Po Town.
Explore Po Town
In Po Town, the Grunts won't take down their barricade.
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 to the right and you have to battle a Team Skull Grunt who has a level 34 Spinarak. You get 1088 Pokédollars for winning.
After that, you have to battle the other Team Skull Grunt. He has a level 34 Grimer. You get 1088 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 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 35 Golbat and a level 35 Raticate. You get 2240 Pokédollars for winning.
Go north from them to find a Rare Candy.
This is a dead-end, so go back through the hole between the Grunts. Then go left 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 34 Fomantis and a level 34 Salandit. You get 1088 Pokédollars for winning.
North of the truck, there is another barricade. Go to the left, and go through the hole in the fence there.
Near the fence, you can battle a Team Skull Grunt who has a level 35 Wimpod. You get 1120 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.