You have to teach HM to Pokemon. After you do that, you need to get the badge that allows you to use the HM outside of battle. If you've already done those 2 things, then you need to press "start," arrow down to "Pokemon," select it, select the Pokemon that has the move, then you will see a couple options. You need to click on the HM move. Then the Pokemon will use it. One more thing. If you need to use the HM for "cut," (teach this to grass Pokemon) then you have to be touching and facing the tree that you need to cut. For "surf," (water Pokemon) you have to be facing and touching water. "Fly:" (flying Pokemon) you can't be in a building or cave. "Dig:" (rock/ground Pokemon, this is a TM move, but still can be used outside of battle) you must be in a cave or building.
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No.
HM 8 doesn't exist in Pokemon Fire Red or Leaf Green.
HM08 doesn't exist in firered.
Waterfall is HM07 in Pokemon Gold, Silver and Crystal only. There are 2 Pokemon in Red and Blue who can learn it naturally (Goldeen and Seaking) however because its an 'old' attack, you may trade over some red or blue Pokemon, teach it to them in Gold or silver and trade them back. This is the only NEW HM that can be taught to old Red and Blue Pokemon, HM06 Whirlpool doesn't exist in red and blue.Waterfull is not an HM in Pokemon Red or Blue. The only Pokemon that know Waterfall in those games are Goldeen and Seaking, which Goldeen can learn Waterfall at level 37, while Seaking is at level 39 in Generation I.
There is no such thing as HM 9 in pokemon, the highest number HM is 8