Answer:
After you won the Vermilion City Gym, go to a house in Vermilion, where it is the Pokemon Club, talk to all the people who are inside he house and they will give you a Key Item Bike Voucher, then you will have to go to Cerulean City Bike Shop and they will exchange yo uthe Bike Voucher for a Bike, then you go to the west of Cerulean City, from route 9 to the Rock Tunnel (you will need Flash) and then you arrive at Lavender Town, you rescue Mr. Fuji, he gives you the Poke Flute, then you go to the south of the city to route 12, then you will face a Snorlax use the Poke Flute then he will battle you he is at level 30 then you continue your journey to the south of route 13, then through the west to route 14 the to south and then to route 15, then go to the west and you arrive at Fuschia City, the you beat Koga, and go to the west to the route 18, then in the Cycling Road you finish it, and then there is a ware house, you go to the top part go to the house and then talk to the people that is inside of the house and h will give you Fly, then from the west you pass to Celadon city beat Erika, go to the Celadon Mansion from the behind take the tea and the Eevee and go to Saffron City, that how you go to Celadon City!!
I hope this helped u very much!