You get it after seeing Groudon and Kyogre fight. Steven will lead you to the Cave of Origin. Go deeper into the cave and you will need HM Flash to proceed. You will find HM 07 there.
HM 07 in emerald is waterfall
HM 05 is flash
A HM is a move is a move that can be tought to as many Pokemon as you like but the Pokemon has to be able.
The Pokemon that can learn any move from TM's and HM's is Mew.
You get HM 8 first from Steven in Mosdeep City (in his house). You get HM 7 in the Cave of Origin (look kind of hard - it's dark in that cave.)
He isn't in Pokemon Emerald. He is only in pokemon Blue,Red, and Yellow. Sorry.
talk to Wallace
H.M
HM02
Zigzagoon and liloone
9
Tropius and Gyarados together can learn all of the HM's.