Memorization of opening moves and how they play out for the first 5-10 or so turns.

After that, a deep understanding of the game. Which will take time and practice.

If you're willing to wait 12 hours for a response - I have a Chess website that will show you the history of your game after playing it and tell you every single mistake you made or when you had a more optimal move to make. It allows you to study your games to improve, unfortunately I don't remember what it's called. I'd have to find it in my IRC logs.

E: I forgot to mention its free and supports playing against other players.