Pokémon FireRed and LeafGreen/Route 19

'''Route 19 is a predominantly north-south water route in southern central Kanto. To the north of this route is Fuchsia City. Its south end is at Route 20 east of the Seafoam Islands.'''

Route 19 is just south of Fuchsia City and is full of trainers and Pokemon. If you're in a hurry, you can fly to Pallet town and take Route 21 to Cinnabar Island instead, but you'll miss out on lots of XP and a chance to catch one of the Legendary Bird Pokemon on the Seafoam Islands. You should take this route now, you can always go to Route 21 afterwards if you want to challenge the trainers for XP.