You simply trade it.
You can't catch or get a Hitmontop in Emerald unless you cheat using an action replay or trade. In Pokemon FireRed and LeafGreen, choose either a Hitmonlee or Hitmonchan from the Fighting Dojo in Saffron City. Then breed the one you chose with a Ditto at Four Island's Daycare. You will get an egg that contains a Tyrogue. Raise this Tyrogue to Level 20 and make sure it has equal Attack and Defense when it evolves. This will ensure its evolution to Hitmontop. Then trade the Hitmontop to Emerald.
TO catch tyrouge you go to some cave some where and fight the karate king
firstget ditto and hitmonchan/lee from pke'mon leafgreen or firered into pok'mon sapphire or ruby then leave them in daycare then a few minutes then go back then the daycare man will give you an egg thenit will hatch into tyrogue
To evolve tyrogue, you have to reach level 20, and if its ATTACK is greater than its DEFENSE, it evolves into hitmonlee. If its DEFENSE is greater than its ATTACK, it evolves into hitmonchan. If its DEFENSE and ATTACK are equal (which is quite rare), it evolves into hitmontop.
In my opinion, Hitmonchan but only cause he can have Thunder, fire, and ice punch You can get a Ditto and breed your choice on Island 4 later on and get a Tyrogue If Tyrogue's attack >20, it will evolve into hitmonchan at level 20 If " defence >20, you get Hitmonlee If Attack=Defense, you will get a Hitmontop at level 20