For all tennis players entering competitive tennis, there will always be a time in the first few games of a match when you will be nervous. Its completely normal. However, its how you deal with that nervousness that will ultimately decide if you will be competitive.
First, realize that the person across the net is probably nervous as well. You are not the only one with nerves.
Second, use your warm-up and practice shots as a way of loosening up. Try stretching and warming up before you step on-court which will help alleviate the physical symptoms of nervousness (tightness, "quicksand feet", etc.)before a match.
Third, keep in mind that you are playing against a tennis ball crossing the net, not the person across the net.
Fourth, play to your strengths during the match, and attack your opponents weaknesses. Use your practice time before the match hitting with your opponent to detect weakness in his/her shots.
Fifth, don't abandon your game if you are a slow starter. Beginners have a tendancy to "go for broke" if they don't start a match well.
Sixth, remember to bend your knees, follow-through on your shots and keep your feet moving. Keep your eyes on the ball and concentrate. Don't be distracted by things outside the court. As the match develops, your limbs will get looser and you can focus more on strategy and breaking down your opponents game.
All of these things I've mentioned take time to develop. Nervousness can be turned into a positive using your mental alertness to focus on the game and not your "head."
In time, you will learn not to be afraid of winning.