I'm totally new to the TES IV construction set, though I'm very familiar with the TES III version. I don't understand how to make a greeting and assign it to an NPC. Can anyone explain?

Start by adding your topic to the GREETING tree.

After typing out the response (maximum 150 characters per line), set the conditions on that topic so that only the NPC you want to say that line will say it (use the GetIsID condition and set the target ID to your NPC).