you go to the house left of the gym (aka Wally's house) and talk to the guy in the red shirt (Wally's dad)
Chat with our AI personalities
just beat your father (gym leader number 5) and wallys father will give it to you
Surf is HM #3. :-)
Get the HM Surf then teach it to a pokemon.
teach it the HM surf
Manaphy cannot learn Surf but it can be taught it via the Surf HM.