Janaka Maharaja did not have any children, so the Brahmins suggested to him to do a hola-yajna.  “You take a plough and draw a line around the palace.  When you move the plough, it will get stuck in the mud, and every time it gets stuck you have to donate gold to the priests. In this way you are giving a lot of charity and you are getting a lot of blessings so you will get children.”

In one place it got stuck, and after giving all the gold, still the plough would not move on.  “There must be some big rock there,” everybody said, so they dug and found a box.  And inside the box was a baby, a female baby, and she looked just like Laksmi.  She was known as Janaki, the daughter of Janaka, and she grew up in the palace. When Siradhvaja (Janaka) was plowing a field, from the front of his plow [sira] appeared a daughter named Sitadevi. Thus he was known as Siradhvaja.

MARRIAGE

Janaka made an arrangement for the marriage of Sita. He wrote a message that read, “If anyone wants to try and string this bow and get my daughter, then please come.” Many princes came, Ravana came, and he put his hands on each side, and he lifted the bow. So then Siva went inside the bow and pressed down.  Ravana got stuck, finally Lord Siva let go. And then Rama came, but he didn’t use two hands, only one.  He put his foot on the other end of the bow and he took the string.  As he pulled it, the bow broke into two pieces.  Everyone clapped their hands, and flowers fell from heaven.  Then Lord Ramacandra got married to Sita.

Source: http://www.ramaiswami.com/appearance-of-sita-devi-2/