Hi,
i hope you don't mind a guy replying but it may help to hear some thoughts from a male perspective.
When the woman is on top the penis needs to be put in to her. Either she can guide it in or he can. If you think about it, when a man is lying down on his back his penis, even if erect, will be lying flat against his stomach. So you need to lift it up in order to sit on to it. When fully erect it will only raise as far as 90 degrees from his stomach (i.E. So it is pointing straight up in the air) but it is more comfortable if it is angled forward slightly. So when you sit down on him make sure you're not sitting to far back and try to lean forward slightly. I'm sure he will guide you in this anyway so don't worry. He won't let you be in any position that is uncomfortable for him.
When you initially sit down on to his penis go slowly and gently until he is all the way inside of you. Once he is all the way in then you can pick up the pace fairly quickly. But if you sit down hard and fast you may cause him discomfort, especially if he has a foreskin or if things are a relatively tight fit.
In terms of what kind of motion you should use, any motion which slides up and down the length of his shaft. So if you were sitting upright on him then it would be an up and down motion. But if you are leaning forward or lying on his chest then it's more of a forward and back motion (keeping in mind that as you sit forward his penis also bends foward inside you).
Things to remember when you are riding him is not to move too far up his penis too aggressively or there is a danger he might slip out of you. This in itself is not a problem, but if he slips out of you momentarily and then you sit back down on his penis and bend it the wrong way it can be mighty painful. So be enthusiastic by all means, but always apply an element of care.
In terms of knowing when to slow your pace, you should be able to tell when he is ejaculating due to the sounds he makes or his facial expressions. You want to maintain the pace of things while he is ejaculating and then quickly begin to slow down once he passes his peak. If you are not sure when this is then ask him to be more vocal with you when he is cumming. After he has ejaculated he will begin to go soft fairly quickly. It is not true to say that it would hurt if you continue when his penis is not so hard, but it just becomes a bit sensitive after ejaculation and can feel a little sore. So when he has cum just ease the pace and bring yourself to a stop.
I hope some of this has helped. This is all from my own personal experience of 'woman on top'. Others may have different advice to give you.
Hope all goes well, take care...