Here are two scenarios for how Rey could be Luke's daughter
1) While training his students Luke meets and falls in love with a woman (possibly one with red hair who used to be an Imperial agent who first comes after Luke looking to kill him *winkwink* *nudgenudge*), and they have a daughter. Luke either learns about or senses some sort of danger, fearing for his daughter's safety he sends her off world with her mother, both now living under aliases. He has Han, Leia, and everyone who knows them swear that they will never tell anyone, including his daughter herself, who she really is.
The danger eventually catches up with the girl's mother, let's call her Mara, just for shits and giggles, and with no other way to keep her safe she gives the girl to Unkar Plunt on Jakku. She is killed before she can tell anyone where the is and nobody ever finds out what happened to her. The girl is left on Jakku until we meet her as Rey in TFA.
2) Luke and Mara meet, fall in love, and Mara becomes pregnant. Mara, who has always been a loner, and never stayed in one place long freaks out, and leaves without telling Luke about their daughter. She has the girl and raises her for the first few years of her life, telling her stories about Lukem The Jedi, the Rebellion and the Empire. She eventually runs into some kind of a danger, and is forced to leave the girl with Unkar Plunt on Jakku, and sets off back to Luke to get help and tell him about the girl, but is killed before she can get there. The girl is left on Jakku until we meet her as Rey in TFA.
That is, on the basic level, what I think might happen.