For people new to dtl, Sian & AW13, I would say (imo) practice until you can hit approx 20 every round (so you score 75-85/100), then enter competitions. If you start shooting comps when you are at this level, you will be shooting decent scores and still be put into C class, which is where you want to be starting out. And depending on how shooting/coaching you get etc, you could be challenging for class wins within 4-6 months.
If you are comfortable shooting in a squad in practice, a comp is no different really. There is an etiquette as you say, follow that and no one will complain (Just dont start off peg 1 for first time out, you will have enough to think about, without thinking about the card too). Maybe shoot your first comp with people you know if it bothers you, but after that just shoot with whoever, as you need to just adapt to different squads, speeds etc. If you can watch a couple of rounds beforehand, so you get used to how a round starts, referee calls etc, that will help too.
But the first comp is the hardest for nerves. Once that is done, you will get more & more relaxed. Comps are where the real fun & satisfaction (and a fair amount of frustration too) is.