how do i find the hm waterfall

In the northern part of sunny shore is a gym trainer from Jhoto (I forget her name but she used steel type) after you beat Volkner she'll give it to you

A good idea in Pokemon games is to talk to everybody, even after you battle them. You never really know what they are going to say or perhaps even give you. Whats the worst that could happen, a Pokemon battle??? Isnt that what you bought the game for? With that in mind in the north part of sunyshore city there is a girl standing there (she is the most northern person in the city) her name is jasmine and she basically just gives you the HM for waterfall. Pretty easy.

David_Banner is correct. In any and all Pokemon games, it is crucial for you to talk to absolutely everyone that you see. You may never know what type of information that you have missed out on, or what type of information you will learn. You will see how this comes into action as you obtain the HM containing waterfall. Go into Sunyshore City and head north, you will reach a small shore with a woman standing there. If you played the old Pokemon Gold, and silver games then you will remember her as Jasmine, the Sixth gym leader of the Johto region. Talk to her to receive the HM waterfall. Hope this helps.

To get HM08 Waterfall, you can go down to Sunyshore City and go as far north as possible to the beach. There should be a person there named Jasmine, a gym leader from Johto. Once you defeat the Sunyhore Gym, talk to Jasmine and she will give you HM08 Waterfall.
Hope this helps!