Chess is a game of strategy. You need to figure out how to minimize your opponents defenses or find any holes in his defenses and take his king. I would suggest watching how he plays and figuring out any repetitions of his defenses and using them against him.
Some suggestions on how to improve your chess game - a book from Bobby Fischer on the basics of chess. He takes players through how to checkmate the opponent and shows how he thinks. You should consider getting chess software to improve your game also.
Beginning chess players will make basic tactical mistakes in every game they play. That is to say, they will hang a piece, or do something that will allow the opponent to force them to lose material. A fast way to improve in chess is to cut down on these mistakes.
There have been books and books written on this subject, but the key is to have some kind of strategy, but don't be so stuck in that strategy that you cannot respond to moves made by the other person. Oh, and don't be so defensive that you forget the offense.