It depends on what game:
Red, Blue, Yellow, Fire Red, Leaf Green, Gold, Siver, Crystal, Rubyand Saphire: Flash
Diamond, Pearl and Platinum: Defog
SoulSilver and HeartGold: Whirlpool
Black, White, Black 2 and White 2: Waterfall
Flash: Normal, any normal type can learn it and some can learn it too
Defog: Flying, any flying type can learn it and only flying
Whirlpool: Water, any water type can learn it and some can learn it too
Waterfall: Water, any water type can learn it and some can learn it too
HM05 Waterfall is on Route 18.
HM05 Defog can be found in the Solaceon Ruins.
hm05 is flash
you need to catch ten Pokemon
HM05: Defog
Route 18
go 2 route 18 and surf. you'll find a ledge. hm05 (waterfall) will be on the ledge.
wich hm is that normally water type can go with surf and waterfall flying type can use fly and grass uses cut but some other Pokemon can use the HMS too
u get hm05defog from a person in the safari zone
HM05 Waterfall is found on Route 18.
go 2 route 18 and surf until u reach a ledge. hm05 (waterfall) will be on the ledge. good luck!
HM05: Defog ( Clears deep fog.)