go to items go to the tms and teach a pokemon that move and then go up to a cuttable bush and press a and it will ask u if u want to cut it click yes and if just keep switching pokemon until you find one that can learn cut
almost any pokemon can use cut, Zigzagoon for for ie. leave a open space for it in your party so you can give all your weak HM moves to it so you do not end up getting stuck with them on your powerful Pokemon so you can get past places or collect items
http://pokemonruby-version.blogspot.com/2008/09/pokemon-ruby-hints.html