When you first enter Douba Road, there will be a guy who gives you a Dragon Fang. If you continue on North, there will be someone who is wearing an orange shirt and will give you a free potion. On the East Side, there is a Trainers School. Enter it to battle your former teacher who has a Level 10 Roselia. After beating her, go outside and move west to Mainland Station Cave.
When you reach Mainland Cave, your friend Pete will battle with you. He has a Level 10 Trapinch. Go into the cave and go to get the package for Professor Ivan. A mysterious trainer will battle you. She has a Level 12 Axew. After beating her, you can get the package.
After getting the package
After exiting Mainland Cave, there will be a guy who you sadly decide to battle. He is another mysterious trainer, but is way better than the last one. He has a Level 50 Tornadous as his partner. You obviously lose and get sent back to your house.
After waking up, you can go over to your Rivals house and get the Town Map. After doing this, there isn't much else to do except catch some Pokemon. So start to make your way towards the Twist Woodlands.