Your friend tells you to go with him to the lake, so you go north from Twinleaf Town to Route 201 to find him.
Go Meet Your Friend
Go north and talk to your friend. After some dialogue, he will follow you. Go west to the next area and you will reach Verity Lakefront.
Verity Lakefront
There will be a cutscene with Rowan and Dawn or Lucas.
Choose Your Pokémon
After Rowan and Dawn or Lucas leave, you must choose a pokémon. You can choose either Turtwig, Chimchar, or Piplup. If you choose Turtwig, your friend will choose Chimchar. If you choose Chimchar, your friend will choose Piplup. If you choose Piplup, your friend will choose Turtwig.
Battle Starly
You will use your newly chosen pokémon to battle the Starly that attacked. Choose Battle, and then you can choose one of your pokémon's moves. You can put the cursor on a move and press Y to find out what each move does. Press Y again to hide the move info display.
Use attack moves until the Starly is defeated.
Return the Pokémon
If you press X, you will see that the menu now has a "Pokémon" menu item. You can use it to look at the pokémon that are currently with you.
Go south and then east until you encounter Rowan and Dawn or Lucas again. During a cutscene, your Mom will give you Running Shoes. You can run by tilting the left control stick all the way in the direction that you want to go.
Go Visit the Pokémon Research Lab
Go north to Route 201, then go east through the tall grass. You may encounter wild pokémon. In addition to the Battle option, you can now select Pokémon, Bag, and Run. The "Pokémon" option will become useful when you have more than one pokémon. The "Bag" option will become useful when you have items in your bag. You can use Run to try to run away from the battle, but it doesn't always work.
If you need to heal up your pokémon, you can go back home and talk to Mom.
Otherwise, keep going east through Route 201 until you can't go farther east. Go north through the large area of tall grass. Talk to the person at the top of the grassy area to get 10 free Potions. You can use these by selecting Bag and going to the Medicine Pocket. They can be used in battle or from the X menu.
Go east from the person who gave you the free potions and you will reach Sandgem Town.