Ascended now

My Monk just ascended, thanks for the advice on getting past the doppelganger, and I am preparing for my first mission. Up to this point I have not used one of the signets (or whatever they are) to unlock an elite skill (or any other kind of skill) from one of the bosses in the game.
My question is how will I know whether to try & unlock a skill upon a boss or not after killing him? Do I have to have the signet already loaded onto my skillbar in preparation for the likelyhood of there being a chance to unlock 1?
Not having done this I am unaware of whether you try numerous times to unlock these skills or if there are just certain bosses that you should try.
Thanks for your replies.

You have to bring a signet of capture on your skill bar.

After you kill bosses (monsters with colored auras) use the signet of capture to see what skills they have. If the skill has a gold border then it's an elite skill and should be captured.

most of the time I run the mission and take note of the skills that ranger/monk bosses are using...then go grab my SoC(if i want the skill) and then re-run the mission..help others get through it and Cap my skill...it's a little time consuming, but I just prefer to do it this way.

You can go and look up what bosses give what skills and in what missions...that way you can pre-load the SoC knowing what skill you are about to get.

You are ready for your first mission? did you get rushed through everything else?

After a boss dies, and it is your proffesion, always use the signet of capture to see if he has anything you need. You use the capture signets on dead bosses. When you use the signet a screen pops up with all the skills you can capture from the boss, you can pick one and get it, or you can choose not to get one, and you will lose nothing.

You buy a signet of capture in town and load it into a slot on y our skill bar like any other skill before you go exploring or into a mission. You can load multiple signets of capture if you want.

After you kill a boss of your profession, stand near the location where it died and use the signet of capture skill by clicking on it on your skill bar. After that, you will be presented with a menu listing all the skills the boss could use. You select theone you want and confirm it. Then the signet of capture will become that skill you just captured permanently. You have the option to hit cancel after being presented with the menu if you don't want any of the skills the boss has. You can end up with multiple elites on your skill bar if you brought one and then captured one. I had up to 4 at once while on an outing to Perdition rock. Once you zone, however, they will be removed from your skill bar.

As for whether or how you know what skill to get, you will want to familiarmize yourself with which boss where uses what elites. You then prepare to go there. You will find people advertising for looking for skill capture groups for this purpose. These are common in the fire island chains and south shiverpeaks where most of the elite skills are. Refer to http://www.xennon.co.uk/eliteskills/ for an excellent compilation of boss and elite skill locations.