When first getting into the scene, it can often be somewhat intimidating to try playing with SM, especially if neither of you have done it before. Here's this wonderful person, who wants you to dominate them. You tie them up, and they're helpless, wriggling with anticipation and lust... and now what do you do?
Play with them! There are all kinds of common objects that produce intense and enjoyable sensations when applied to a helpless lover. Combs for dragging across the skin, feathers for tickling a frantic foot, clothespins (use a couple or use many in artistic rows--these can be as intense as you want to make them!), ice cubes, chocolate syrup, strawberries (ever see 9 1/2 Weeks?), leather belts for slapping or spanking, hairbrushes for scuffing or beating, and of course your own fingers, mouth, genitals, and everything else. Enjoy taking your time with your willing victim; drive them to distraction, then bring them to the edge of ecstasy, then back off and make them beg for more!
Remember, you can set the mood as you wish. You can be playful, amused at your own ineptitude even while your bottom is moaning with desire. You can be stern and commanding, sympathetic but nasty, jolly yet sadistic--anything you please. As long as you focus your attention on your bottom, your bottom will have a great time! Relax, go with the flow, and if you stop enjoying it, call safeword--tops can use safewords too.
Of course, there's no need to feel like you need to put all your attention into pleasing your bottom; what's a good slave for if not for pleasing their master? I've several times played with my girlfriend and used her for my own selfish pleasure, giving little attention to hers--and she loved it! But there is no doubt that with pleasure it is as good to give as to receive. Just remember, communicate, be sensitive to what your bottom is feeling, and you'll have no problem.
The one thing that is quite important to remember as a top is that you are responsible for your bottom. As you begin playing with S&M, you may well be placing your bottom in situations in which he or she is physically helpless and/or emotionally vulnerable. It is important that you recognize they are placing a great deal of trust in you, and in your ability to handle any situations that may come up. If you're in the middle of a hot scene, and suddenly someone unexpectedly bangs on the door, you may both be startled and shocked--but your bottom will be immediately looking to you for protection. If something happens that you didn't expect, take care of your bottom _first_--reassure them that you're not going to let anything happen to them, and then deal with the problem calmly and sensitively.
And don't be limited by preconceptions of what you "ought" to be doing, or worries about how you're not topping "correctly." If you start to feel pressured or insecure, take a step back, and ask yourself what _you_ want out of the scene. Sometimes, when I've been bottoming for my girlfriend, she hasn't been in the mood to play with me sexually--so she made me her slave and commanded me to... bathe her and wash her! This was lots of relaxed fun for both of us, and it let her unwind enough to keep playing the way _she_ wanted to play. Be honest, not only with your bottom, but with yourself. And if you are in the middle of a scene, and suddenly your honesty says "I don't want to be doing this" or "I don't know what my partner wants, or even what I want," then by all means stop the scene--gracefully if possible. Better that than for the scene to drag on until both of you are sick of it.
If you still don't have any ideas, and if your bottom is really hot to trot, you can always start playing with your bottom and getting them excited in whatever way you know how, and demand that they tell you a fantasy of theirs, or you'll stop. Talking dirty to each other--trading hot fantasies, knowing that you can make then happen if you want to--is the best way I know of getting ideas for scenes. This actually goes for ALL sexual play, whether it involves S&M or not!
If you want detailed descriptions of positions, possible scenes, and so forth, you would do well to find a copy of _Sensual Magic_ or _SM 101_. See the resource lists on the "Sources" page (and order some mail-order catalogs of S&M books; lots of ideas!). Or, post with your questions to the Usenet newsgroup soc.subculture.bondage-bdsm, asking for any and all suggestions.
...check the menu for tips on "How to be a good bottom."