I talk to many people who are curious about the dominant side of BDSM, and the thing they're most nervous about might surprise you. Perhaps you imagine fledgling tops being primarily concerned with getting what they want out of a scene. That's certainly important, but what seems to trouble novices more is performance anxiety. Or, as someone recently wailed to me, "How do I know if I'm doing everything right?"
First of all, breathe. None of us popped out of our mother's womb with a flogger already in our hand. We're all beginners at some point. My advice is this: Don't measure your prowess in a BDSM scene by the intensity of the sensation you dish out or by the complexity of the role-play you invent. Whatever the scene is, measure its success by the response you call forth from your partner. For that, there is a simple grading scale, which I learned years ago from BDSM educator Joseph Bean.
There are three levels of success for the top in a BDSM scene. The best possible outcome: After the scene is over, the bottom wants to do that particular activity again, with you, in the future. If that's the case, then you did just fine.
The next step down the ladder: The bottom wants to do some kind of scene with you in the future, though he/she doesn't want to do the same activity again. That's not an uncommon outcome, and it's not necessarily anything to get upset about. It might mean you need to polish your technical skills, but your overall style is fine. Or it might mean the bottom wouldn't like that activity no matter who did it.
The third-best outcome is when the bottom wants to do some sort of BDSM in the future with someone else—but not with you. This is success only in the sense that you didn't single-handedly turn someone vanilla. Faint praise, indeed.
But if none of these statements are true? You failed as the top in that scene. (It is true that if you play with enough people, sooner or later someone will do one scene with you and decide kink isn't for him/her. So one must consider one's average score as well as the outcome of each individual game, and not hang up the restraints if it's one out of hundreds.)
Now, I could give you some touchy-feely bullshit about how no one ever really fails as a top. But that's not true. It's entirely possible to be a lousy top and have your scenes fail. This is not a game where everyone's a winner. However, a scene might turn out quite differently than you planned and yet still be very enjoyable for your partner and you. So keep this yardstick in mind. It will help you accurately gauge BDSM success versus failure in the only arena that matters—the bottom's head.