Instructions
Travel south on Route 1 to Pallet Town. You will encounter several trainers on this path; battle them to earn valuable experience points.
Use a Pokemon with the "Surf" ability to get onto Route 21. This is another hotspot for battle and experience gaining. Go south on this route and you will come to Cinnibar Island.
Head to the next road, Route 20, from here. You will need "Surf" again to traverse this road.
Continue right on Route 20 until you reach Seafoam Island. Climb a staircase to reach the gym, where you can battle the boss Blaine and earn the Volcano Badge. You can continue right onto Route 19 to battle some more characters before heading back to Cinnibar Island.