There is a quest on fleet for choosing your AC, as quest reward you should get an backpack with the right Lightsaber.
The normal quest route was completing your Lightsaber, doing one or two quest then go to fleet and choose your AC, after 4.0 you can choose your AC simply at lvl 10, but you can still do the quest.
Can you still do the old Advance Trainer quest part? The Bounty Hunter one was always interesting I thought. But Ahrngrim is right you should have a backpack item in your inventory after selecting your advance class. It'll have a double bladed saber and probably a offhand focus/shield/generator or a couple to choose from. If you don't have it in inventory you can get some orange lvl double sabers at the level 8 vendor on fleet or capital planet or GTN will have some. But you can continue playing with just the standard blade until you receive a new blade as a quest reward. Some of your abilities won't work until you get the double blade but you should be good running with companion until then.
