1. how do i do custom dialogue?
2. how do i make custom travel services?
3. how do i make a vendor and choose his inventory?

edit->4. i am working on my first large mod (underwater city) and i would like to know how i fill interiors (with furniture, beds, containers etc) with water



thanks in advance

#1: http://www.fliggerty.com/phpBB3/viewtopic.php?f=63&t=4463. That should get you started on making your own custom dialog.
#3: Make an NPC and set what he can sell under the AI tab. If you don't want to add stuff to his inventory (he'll sell anything in it that he offers like weapons), put some crates in the cell he's in and assign him ownership of them. Remember that vendors carry everything they buy in their inventory, but they'll sell anything assigned to them whether it's in a crate, on a shelf, or in their inventory.

1. There's too much info for a single post. Read some of dialogue tutorials from http://www.gamesas.com/topic/24-comprehensive-tutorial-and-useful-link-list/.
2. Open NPC's window, click "AI" button, select "Services" tab, select a cell where you want to travel, click "Select marker" and move the marker in the render window to the exact position.
3. In the same "Services" tab, select a class of items you want him to sell. Then either add whatever you want him to sell into his inventory or into a container somewhere in the cell and then double-click that container in the render window, select the "Extra data" checkbox and set the owner.
4. Menu->World->Interior Cell->Has water.

Thank you