Few random pieces of advice for you...
Get all the kit you'll be wearing in your fight together as far ahead of time as possible. Any gumshield, shorts, cup, strapping, kneepads etc etc. Train/spar in them as much as possible leading up to your fight. You don't want any surprises from your kit on the day of the race; the experience will be foreign enough as it is without adding more odd new things to your day.
Don't let every man and his dog be milling round you in the dressing room before. Have as many or as few people around you that make you feel comfortable - no more, no less. Often lots of people who don't know the fight game mooching about asking you questions like "aren't you afraid you'll break something", or "what if you get choked out" aren't conducive to the right mindset.
When you get in the cage or ring, look around, breath in and smell the air, listen to the crowd, feel how the mat feels under your feet. Basically try and get all of your senses used to where you are. It can feel other-worldly the first few times in there and the more you let your body know this is real, the better.
Breathe. Seriously, I've forgotten to do it on a number of occasions. "Tom, fucking breathe, you've gone purple!!!"
Most importantly, enjoy it, relax, and don't get too hung up on the result. Of course you want to put your heart and soul into getting the win, but the reason you have Amateur and Semi Pro fights are for ring experience, not to build up some fearsome 10-0 record.
All the best.