How do you get in to celadon city in Pokemon Red?

Answer 1

When you get in Lavender Town, go west and you will find a tunnel. That tunnel leads to the Celadon City. You will only get the silph scope and finish the gym there then go back to Lavender to get the Poke Flute.

Answer 2

Go to the route west of Lavender and take the underground tunnel at the end (it will look kinda like a house). Once you're out keep going west and you should reach celedon.