go to Viridian city then go to into diglet's cave. at the other end there will be small trees that u can use cut on. do that about 2 or 3 times and u should find it.

if you got cut go to viridian forest's entrance not go in there is a house don't go there
too there is small cutable tree right of the pokemon bushes.cut it and go up and down
you'll find a house.there is PROF.OAK's AIDE he will give you flash HM if you catch 10 or
20 ?? kinds of pokemon.