HMs are very important to get through the game. You need every one (except Defog) to get through Sinnoh. Here are all 8 HMs, where to get them, and what they can do.
HM 01 - Cut
Cut can be obtained from Cynthia in Eterna City. It can be used to chop down small trees. Cut is not a very strong attack, so I don’t recommend using it on Pokémon you will be battling with.
HM 02 - Fly
You can get the HM Fly from the Team Galaxy Warehouse. It can be used to fly back to any city that you’ve been to. Fly is a decent attack to use on any Flying Type and I like to use it on any Pokémon I can while I’m playing the game.
HM 03 - Surf
Surf can be obtained at the Celestic Town Shrine. It can be used to travel on water. Surf is one of the best Water attacks in the game and I like using it on most Water Types.
HM 04 - Strength
You can get Strength from the top floor of the Lost Tower on Route 209. Strength can be used to move Big Boulders in Sinnoh. Strength is not one of the best Normal Type Attacks, but it can be used effectively on some Pokémon.
HM 05 - Defog
Defog is one of the most useless attacks in the game. You can get it (or not get it) from a person in the Safari Game. It is used to get rid of the fog in one Route but you don’t really need it.
HM 06 - Rock Smash
You get Rock Smash from a Hiker at the Mt. Coronet entrance. Rock Smash is used to smash away rocks that are in your way. It is not a very strong attack.
HM 07 - Waterfall
You get Waterfall from Jasmine in Sunnyshore City. Waterfall is a good Water move to use on Pokémon with high Attack.
HM 08 - Rock Climb
Get Rock Climb on Route 217, because you’ll need it. Although Rock Climb isn’t the best HM to use on a Pokémon for battling, it isn’t bad. But still, I don’t recommend using it.