"When will I wed the prince?" she asked.
"Never. You will wed the king."
Maggy the Frog clearly predicted that Cersei would marry Robert, not Rhaegar or any other Targaryen.
... "I will be queen, though?" asked the younger her.
"Aye." Malice gleamed in Maggys yellow eyes. "Queen you shall be . . . until there comes another, younger and more beautiful, to cast you down and take all that you hold dear."
It's generally agreed that the younger, more beautiful queen must be either Margaery or Daenerys. Against Margaery is the fact that she is not, perhaps, more beautiful -- everything indicates she is pretty, but probably not the beauty that Cersei is (or was). Daenerys, on the other hand, certainly has the almost "inhuman" beauty of the Targaryens. Against her, on the other hand, are the events of A Feast for Crows, where Cersei does already seem to have lost most everything she holds "dear.". However, as the next prophesies show, she's not yet lost everything, and Margaery Tyrell is much less likely than Daenerys Targaryen to put paid to those.
... "Will the king and I have children?" she asked.
"Oh, aye. Six-and-ten for him, and three for you."
Maggy the Frog has predicted the total number of Robert's bastard children, and Cersei's children as well. On a tangent, the theory that Cersei is pregnant loses a little of its spring; we know the three children she has. If she is pregnant, she'll not carry the child to term, at least.
The old woman was not done with her, however. "Gold shall be their crowns and gold their shrouds," she said. "And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you."
Maggy predicts that Cersei's children will not survive her, but the interesting part regards the crowns and shrouds, which strongly implies that they shall die as kings and queens. This has already come to be true for Joffrey. Tommen's death certainly seems inevitable, all considered. How Myrcella will end up crowned given the events of A Feast for Crows is an often overlooked question.
Finally, there's much speculation as to the valonqar -- which we learn means "little brother" in High Valyrian. Cersei fears that it is Tyrion, but it seems entirely possible that it could be Jaime, given events in the novel. While he does not have both hands any longer, he does have a false hand, and no one says that the prophecy has to be a hundred percent accurate.
"I get three questions too," her friend insisted. And when Cersei tugged upon her arm, she wriggled free and turned back to the crone. "Will I marry Jaime?" she blurted out.
You stupid girl, the queen thought, angry even now. Jaime does not even know you are alive. Back then her brother lived only for swords and dogs and horses . . . and for her, his twin.
.... "Not Jaime, nor any other man," said Maggy. "Worms will have your maidenhead. Your death is here tonight, little one. Can you smell her breath? She is very close."
As we learn from Cersei, Melara Hetherspoon "fell" down a well and drowned soon after (IV: 413). This piece of language suggests it was an accident. However, other details, such as Cersei's recollection of how she "screamed and shouted" in the well (IV: 583-584) and Maggy's uncanny accuracy, make it seem much likelier that Cersei pushed her down the well herself. The reason as to why she would do such a thing is clearly anger over her pretensions to Jaime's hand, made clear from the middle paragraph, and from an earlier remark in which she claims Melara proved to be a "greedy little schemer..." (IV: 356) Also, it may relate to Cersei's recollection that Melara had convinced her that if neither of them spoke of the prophecy, it wouldn't come to pass (IV: 544). Cersei may have decided that she could not trust Melara to hold her tongue.