Awareness: Take the signals that your body sends you and be aware of what is happening around you at that moment.
Breathing: Breathe deeply and slowly, trying to relax as much as possible. Unfortunately, this is very difficult at the beginning, but incredibly important. Exercise is clearly the master here as well.
Thoughts: Negative thoughts are counterproductive and numb you. They never let you get into action. Handle before you begin to ponder and thereby gain an advantage. Do not get your thoughts under control, then run away. Generally you should avoid a fight whenever possible. A avoided fight is a won fight.
The Right Training: Neither the best book nor this post will ever be able to perfectly prepare you for the fear of the first fight. Just as you can not learn to swim by mere theory, you will not learn how to control your fear by avoiding it. Train as realistic as possible and increase this level from time to time. Through sparring, get to know the feeling of being hit and gain sufficient experience, even before you ever get into the situation of having to defend yourself.