In John 14 Jesus tells the disciples that he was going to prepare a place for them. Isn't it that heaven was already founded from creation? So, what place was he referring to? Was this just a figure of speech?
Technically, I don't know of any place in the Bible that says that the saved go to Heaven when they die. It says they go to "paradise", that they go to be "with Jesus", etc, but I don't know any verse that says this is Heaven. Exactly what the place where the saved go is called is not entirely clear.
Perhaps the place that Jesus went to prepare is the New Jerusalem of Revelation.
Or if it is the same Heaven mentioned throughout the Bible, this would not really cause a problem with this verse. To "prepare" a place does not necessarily mean that you are creating it from nothing. He might have meant that he was preparing Heaven to receive the saved. Like someone might say, "Oh, I have to get home to prepare the house for the party tonight." They don't mean that they're building a new house, but that they are going to decorate their existing house, arrange furniture, put out clean towels, etc.
Jay's answer has got the general idea right, but the meaning of what Jesus said is much deeper and much more significant. I first heard about this in a sermon earlier this year, but you can find corroboration of this online (like here).
In Biblical times, the Jewish custom of marriage was such that even after the bride and bridegroom were married (after the bride accepted the cup), the bride didn't go and live with her "husband" nor did they build a new house and move there. Rather, the man went back to his father's house and built a room that added on to his father's house. When the room was finished, the husband could go back and get his wife. However, it was the man's father who decided when the room was done, when it was satisfactory. Once the father finally said that the man's room was done, then the man could go and get his bride with much fanfare, bring her back to his house, and they would have a feast - the wedding feast.
Sound familiar? It should! Jesus offering the Cup of the Covenant is His proposal! However, His bride couldn't go and be with Him just yet; He had to prepare a place for us first.
When His Father finally tells Him that everything is ready, then He'll come back and get us.
When the rooms are completed, Jesus will be released to come and get His Bride - the Church!
The exact identity of this place (God's mansion) isn't explicitly given in the Bible, but it's most likely the New Jerusalem (where 20 billion inhabitants could each have a room that was a cube half a mile long, wide, and high). Heaven may have been one of the first things God created, but by no means does that mean Jesus can't continue creating, either in Heaven or on Earth. If anything, God consistently and frequently shows that He likes to be intimately involved with His creation, fiddling with the little details and enjoying watching things unfold.
So, to directly answer your questions, Jesus was talking about Heaven, and He wasn't using a figure of speech at all. He meant it literally.