The Slumbering Weald (pronounced “wee-ld”, like “wheeled” or “wield”) is a foggy, eerie forest. Hop will go ahead of you, leaving you free to walk into the tall grass. You’ll sneak in the grass and should encounter a wild Skwovet at level 2. You don’t have any Poké Balls to catch Pokémon with, so you’ll only be able to fight or run from any wild Pokémon you encounter.
Keep going and find Hop. You can talk to him to heal your Pokémon if you’d like.
Follow the path to the west, across a bridge and through some more tall grass, fighting any Pokémon you’d like. Eventually, you’ll hear a cry. Continue onwards, following the path until it curves north and then Hop will run forward into the fog.
As you continue deeper into the weald, the fog intensifies a lot, making it harder and harder to see. That’s when you’ll see a mysterious Pokémon emerge from the fog — and challenge you to a battle. Its name will just be “???”.
Your moves have no effect on the Pokémon. Keep using them anyway. A few turns will pass and the fog will cover up more and more of the battlefield until it’s to the point where you can’t even see the screen.
You’ll exit the battle and Leon will call out for you, coming to your rescue — although the Pokémon that caused that disturbance is no longer around. Thankfully, after a bit of discussion, you’ll see that the Wooloo you went into the Slumbering Weald for is alright.
After some more talking, you’ll be brought back to Postwick. You need to say goodbye to your mum before you can proceed any further, so go and do that and she’ll give you 
30,000. Nice!
Now you can go meet Hop back on Route 1, except you’ll be able to properly explore it now that you have a Pokémon.