first, find the HM cut. then teach it to a Pokemon. then take that Pokemon to the tree and go on the menu option 'Pokemon' the select the Pokemon, then select cut. or insted of doing the menu, 'Pokemon, etc, just press a when standing directly next to the tree (and facing it). for more info go to http://bulbapedia.bulbagarden.net/wiki/Main_Page (copy and paste) and search for 'cut (move)'
this only works on the saplings that when you press a, without a Pokemon that knows cut, will say "this tree looks like it can be cut down."
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get it from a honey tree
Teach HM Cut to a Pokemon and use it on the tree.
I think you have to cut down a tree
Pokemon diamond to find the TM thief go to Eterna City, go to the team galactic building cut down one of the trees go to the right (your right) go over to the side where that other tree that can be cut down, cut it down and walk as far as you can and you'll find a pokeball thing press A and you will then have thief on diamond version
a honey tree