You first beat the fighting gym and choose Hitmontop of Hitmonchen, it doesn't matter. Then go to the 4th island and breed the Hitmontop or Hitmonchen with a Ditto. Then llater you will get an egg. Hatch the egg by running around a bit. The pokemon will be Tyrogue. If you raise it to level 20 and the attack and the defence are the same then Tyrogue will evolve into Hitmontop.

go to saffron city in kanto
beat the "fighting gym," i don't mean the sabrina one
u get to pick hitmonlee or hitmonchen
breed with ditto at daycare center