1. How do you talk? I think I know, but I'm not sure...
2. How do you know if someone's online? Is their name in yellow?
3. If not, then what DOES the yellow name thing mean?
4. How do you know who is who? I have no idea where the vehicle vendor guy is and I really want a vehicle...
1. Chances are that no one at your physical location is active/online at the time, so the best way to communicate with other players is just through conventional means such as e-mail, ICQ, IRC, and forums. Most active players spend the majority of their time on board their personal ships, so it'll be difficult if not impossible to contact them in game.
2. Only way to tell is if their name appears in the Online list linked at the top of each page (The word 'Online' in '123 Online' or whatever the number may be at the moment).
3. I believe you are referring to tutorial NPCs, which display in yellow on your scanners. If you hail them you have a few options to talk to them and learn a bit about SWC.
4. Should be clearly marked in their names; I believe the vehicle rental NPC is called "Speeder Rental" or something similar.