You have to find the HM Cut and use it to teach your Pokemon cut. Not all Pokemon can learn it, but you can use it over and over again. The HM is in different places depending on the game. In the gameboy colour games you get it from the captain of the SS. Anne, in ruby/sapphire/emerald you get it from this guy in a house in Rustboro city........
You can use the move cut once you have defeated the second gym leader
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
In which Pokemon game?
The second gym leader is brawley of Dewford town. You will come to the PetalburgGym second, but that's the last gym leader you'll battle.
The second leader is the town Nacrene City. But watch out she is tough, and she has the second evolution of lillipup and a watchhog. Watch out!
walk in the gym
eterna city
all of them espesially snorelax
gardenia - grass type
Dewford town.
Find cut
Beat the Gim Leader.