Requires some hassle.
-You must set up a quest in the quest editor.
-You must make a new npc. (Better copy an existing one, like Barnabas)
-You must make a conversation in the conversation editor, and assign the conversation file to the appropriate field in their file. (And quest ID if you want quest markers)
-You must append the quest in the world editor for it to finally work.
(Auto-building assets between each step and such I did not mentio as I consider them given)
Why don’t you download a simple mod and reverse engineer it? The modding tutorial mod is quite good for this. Got you nowhere I noticed just now.