Do you mind giving more information about the situation? You should never fight if you don't have to unless your 100% sure you are going to win. Or else you usually lose, unless the other person is plagued by the same doubts as you then it's 50-50.
Regardless to answer your question itself there's no way you'll be able to prevent your body from producing adrenaline. Without the adrenaline you'd also probably be screwed since fights, especially not for sport which I doubt it would be since your nervous, are all about aggression when the combatants are untrained. When you watch 2 untrained fighters fight that are relatively evenly matched the person who is more aggressive confident and keeps their punches tight and attack vicious fast & relentless almost always carries the day barring a lucky shot. Practice the spin backfist. It's a very simple technique (although so dangerous it's banned from basically everything but MMA, you can't use it in a sanctioned martial arts fight even as a high ranking grandmaster.) to quickly get the hang of and has a ridiculous amount of power and most people in a fight aren't expecting it especially not out the gate. IF I were you, and I had to fight, I would quickly learn that move research how your supposed to have your guard and how to actually block punches then go out and start by hitting him with the spin backfist. If he's in tight or charges you you can also do the same thing but spin elbow and just use your elbow instead of your fist.
It takes years of practice to control your adrenaline levels to them remaining relatively normal in a fight.