Move the king two spaces over to the side you are castling on, and then the rook will go over automatically.
castle


- The king must never have moved;
- The chosen rook must never have moved;
- There must be no pieces between the king and the chosen rook;
- The king must not currently be in check.
- The king must not pass through a square that is under attack by enemy pieces.
- The king must not end up in check (true of any legal move).
- The king and the chosen rook must be on the same RANK.

I recommend that you take a look at http://www.chess.com/learn-how-to-play-chess.html as this could be very useful for you as a beginner.
I'm a beginner, how do I castle during a game?