I think first you should have an absolutely cold head during the fight, you should not let your emotions control you, you should always have control of what are you doing.
I understand that you will probably be very nervous, but its extremely possible that your opponent will be nervous aswell. And I cant say that nerves are neccesarily a bad thing, but you should control them.
Nerves are like fire, if you have the control then it will be helpful, but if they control you, it will end very badly.
Even if for example you are at a bad position you should try to not panic and rest calm. Its obviously very easy to say that, during a fight its a completely different feeling, but still try to remain as calm as possible.
As other people have said, listen to your coaches, have confidence on what they say to you.
You should be mentally prepared, do not expect an easy fight, all the contrary, expect your opponent to be very tough. You should not be afraid, but you should never think "ah, probably my opponent wont be that good". Many people think like that and when they fight and they see that the opponent is actually not bad at all then psycologically they are dead and cant do anything.
Expect a tough fight but dont be afraid.
Stick to the game plan, dont try to do flashy things.
Also try to have fun, I guess that you do this because you like it right? Then enjoy it.