Route 6 is a short, grassy path in central Kanto. It connects Saffron City in the north with Vermilion City in the south, but the first time you visit you won’t be able to reach Saffron. You’ll take the Underground Path from Route 5 to Route 6.
Walk to the northwest corner of the route before anything else and grab a Super Potion. Now you can challenge the different Trainers along the route. A boy and girl Trainer can be found near the Underground Path exit.