Have one of your Pokemon use the move Cut.
One of your Pokemon must know cut and then you go up to the tree and press a to use cut to cut down the tree
using cut
If there is a pokemon in your party that can use Cut and you have the second badge then face a tree that can be Cut down press A on it then press A on YES to cut the tree alternatively you can open the pokemon screen and select your pokemon and press A on Cut.
You simply teach your Pokemon cut, go on start, select Pokemon, select the Pokemon you tought cut, and then select cut at the tree u want to cut down. You simply teach your Pokemon cut, go on start, select Pokemon, select the Pokemon you tought cut, and then select cut at the tree u want to cut down.
Teach HM Cut to a Pokemon and use it on the tree.