Hello.

If I want to make a diaolog between an NPC and the player, And I want that in the end of the diaolog the NPC will take something from the player and give him something. and also, If the player don't have the item that the NPC wants to tale from him, The NPC will tell him "You don't have X".

How do I do it?
Thanks for helpers

Start here:

http://www.creationkit.com/Bethesda_Tutorial_Dialogue

I just want to add a simple dielog, not a quest with a full questline. Cant you just explain me here? I want a marchant to say want I want.

Well, you have to make a quest in order to make your merchant say what you want.
There are of course other "dirtier" ways to do it, but if you want your mod clean, the best way is to create a quest for it.


You'll only need to read the first part of that tutorial in order to understand how to set up dialog, no point in me copying and pasting the instructions from the creation kit here.
As for the fragment you have to add to your dialog in order to make the player receive something from the merchant, it would go like this:

Game.GetPlayer().AddItem(MyItem, 1)
akspeaker.RemoveItem(MyItem, 1)
Game.GetPlayer().Removeitem(ThatOtherItem, 1)
akspeaker.Additem(ThatOtherItem, 1)