Ok, so on page 5, Setup Section 5. “ Distribute Title Cards: ” it says to give the President title card to Laura Roslin, Gaius Baltar, or Tom Zarak, in that order. The Admiral title card goes to William Adama, Saul Tigh, or Helo Agathon, in that order.
My question is; if no one chooses any of those characters…do you just follow the line of succession on page 28? Or start with neither President nor Admiral, and get one as the game progresses?

You follow the line of succession. Highest character on the list of succession gets the title.