How easy would it be to set it up if, after a banter with an npc, a door gets triggered to open, which transports the player to what the door was linked to?

If you know what your doing, pretty easy.

In the result script attached to the dialoge, you 'unlock' the door in question. That assumes you initially locked it, requireing a key that cannot be had.
It does not stop consoling the door open, but there are more complex ways to stop that.

You could also have two sets of doors, with one disabled that 'goes' where you want it. And a second door enabled that goes nowhere. In the result script, you either enable or disable the doors properly.

But what if you don't know what you're doing. >.>

Basically, I have a door that works, an npc that works, a banter that works, quest stages that work, but have no idea what coding needs to be put in the banter to trigger the door.

Depends on what you want to do. Look in the WIKI tutorials for the Lock or Enable functions.

Lock: http://www.creationkit.com/Lock_-_ObjectReference
Enable: http://www.creationkit.com/Enable_-_ObjectReference
Disable: http://www.creationkit.com/Disable_-_ObjectReference