to catch heatran, you have to have done these things:
1)obtained the national dex
2)beaten th elite four.

 and you need these things:
1)pokemon on your team knowing surf, rock climb, rock smash, strength, and fly(a time
saver)
2)escape rope, max repels(lots), lots of any type of pokeballs,and a master all(if not
used) 
3)strong water type pokemon(in stark mountain, there are mainly rock,groung and fire
types)
4)recovery items(max potions, full heal, hyper potions, full restores, herbs, e.c.t)

 and to catch him, follow these steps:
1)go to the battle area
2)on the left side of the battle area, keep going up, passing the obstacles and beating
all the trainers till you get to a place known as survival area.
3)go right and again pass all obstacles, and then surf when you go to some water, and when
you get to the other side on some land with a lot of grass and a door, that's it.
4)go up and go past all obstacles, getting to where someone called buck talks to you and
goes away.
keep going up, then right, down, up and up some steps and you should be outside a cave to
the left.
5)go inside the cave, and go through all obstacles until you reach the other side and see
another entrance/exit/doorway, and there should your rival near it, and talk to him.
6)go through the door and keep going straight until you buck comes up to you and talks to
you.
7)he joins your side and then you have to search the whole place until you get to the
other side and when you get to a cave doorway on top of some stairs, buck should start
talking to you, if he doesn't, search some more.
8)buck will talk to you when you try to go into the cave and he should go inside before
you do.
9)when inside, buck will go to a rock in the middle of the room, take it and a earthquake
will happen, and he will go off.
10)use escape rope and fly off to the survival area, go in the house to the left of the
pokemon centre and talk to buck.
11)go back to stark mountain, and into the room with the rock that buck took.
12)heatran should be there, so save the game.
13)battle heatran and off the game to try again if you kill him or for whatever reason.

 quick tip: heatran will be at lvl 70 and knows the following moves:
lava plume
iron head
fire spin
scary face
it's REALLY a good idea to save the game every now and then and also before you fight
heatran or any other lesson.
P.S, heatran is very strong and also very weak in a way, so use a strong pokemon with high
HP but also use average attacks with around 80-120 power.
hope this helps