You have to go to Mt. Mortar and find the dojo sensei mentioned in Saffron City. The guy there says he's out training, and he does that in Mt. Mortar. You'll need a few HMs, you'll want some repels, and you'll want to keep an open slot for the tyrogue. (That'll save you a lot of frustration) Just beat him in a battle (see the video on how to get there), and he'll give you the pokemon. To evolve it, get him up to level 20, and depending on his stats, he'll evolve into one of the three hitmons.
If attack>Defense, he'll be a Hitmonlee.
If Defense>attack, he'll be a Hitmonchan.
If Attack=Defense, he'll be a Hitmontop.
To get the two others, just breed him in the daycare with a ditto and you can get another one. EV training or the vitamins will help you to get a specific pokemon. Don't forget to press B to cancel if you're having trouble.
http://www.youtube.com/watch?v=BItcrgzR9I8
