There's different ways depending on the BC.

There's some BC's in the original areas pre Zilart, which require an Orc, which can be bought using Beastmen seals or Kindred Seals from Shami in Port Jueno. These BC's are mostly for money and items.

There's story BC's in all areas (RoZ, Original, CoP, ToA and the like) which are story based on the missions. Much like the dragon BC for 2-3, where you had to do all the missions leading up to it to enter and fight. These are usually my favorite BC's. >.>

There's ENM's which are CoP only area BC fights. Which require you to talk to an NPC before hand to get access to the BC fight. These are done for EXP and money. And require nothing but the quest to do, and can be repated, but only once every 5 real days.

Then there's finally ISNM's which are the new BC's added into ToA areas only. You talk to some NPC in White Gate and spend so many credits to get orders to enter the BC area. Then you just pick the area to go to, and fight it. I think you were limited on how many you could do every day though. I don't know too much about these types yet.

And well that's pretty much all the BC info you need to know.