You have to get the hm, then, you have to beat bugsy, the Azalea town gym leader. After that, teach it to one of your Pokemon so you can cut down trees.
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bidoof and bibarel can cut trees
You get HM01 Cut from Ilex Forest. Use that to cut down trees.
HM01 Cut
Teach HM Cut to a Pokemon and use it on the tree.
You have to get the HM Cut from the Captain of the ship in Vermillion City.