Here's the scoop. If you level your BLM to 18, you can then have a subjob. If you just choose a subjob, you'll be a Level 18 Blm / Level 1 (otherjob).
You have to *level* your subjob, and your subjob does NOT gain experience (you have to take it as your main job to level it). So, While my Black Mage is level 24, now, my White Mage is only level 8. So, I can be Either a 24blm/8whm OR an 8whm/4blm.
AdvJobs (Sum,Rng,Sam,Nin,Drg,Drk,Pal): You cannot even SELECT these jobs until you're done a quest to "earn" them. You cannot complete Any of these quests until your main job is level 30. At that point, you STILL have to choose it as your main job, and level it from level 1. So the minute yu're a summoner, you're a level 1 summoner / 1 blm. =) Hope that explains it!