To explain it a bit further..
Once you get to level 30 in any base job.. you can then do quests to get access to advanced classes.. dragoon, bard, samurai, etc..
Each advanced job quest is different, ranging from killing 100 monsters to killing a lvl 54 dragon, to running around delivering things..
The bard job isn't particularly hard. You don't need to fight big mobs to accomplish it.. but it will take time.. running around doing the two main tasks. If you click above on the "Bard Flag Quest" at the end of the description it will tell you how to go about the quest.
But as far as people saying you need to level your desired subjob first.. that's not entirely something you need to be concered with but you should consider it.
Now at the moment, I'm a galka 24th lvl mnk and 10th lvl war and 5th lvl whm.. What am I going to do? Well.. I was thinking dragoon.. but I'm getting sorta bored with being a damage monkey and tanker.. it's fun.. but i wouldn't mind being more of a support person. I'm very seriously considering going to brd/thf at 30. That's right.. I'll have to level my sub job as I go... or maybe just before.. who knows..
Maybe I'll sub with war at first to have some combat abilities.. then level up my thief as I can.. It doesn't take more than an hour or two to get to 5th level in any job.. except maybe bard.. cause you can't solo for crap... from what I hear.. that may or may not be true..
I spend a couple hours as a thief.. I'm 5-7th.. That covers me till I get to 10-14th bard.. which will take a great deal longer than 2 hours.. :) but hopefully not too long. :)