Good luck in your tournament
First, toss for choice: serve, side, defer.
Then, typical warmup includes baseline rallys for a couple minutes. Then one person comes to the net takes some volleys. They finish by asking for a few overheads. Once they go back to the baseline, the other person comes to the net, takes some volleys then asks for some overheads.
Once both players (or all 4 if doubles) are done at the net, take some serves. After serves, begin. Enjoy.
This is just a warm-up, hit nice balls back to your opponent, don't go for winners or crush forehands at them.
Personally, the 5 minutes isn't enough for me, so I typically try to find someone to hit with before the match. Not always possible and you probably will have to hit at a different location as the tournament site will most likely have matches on all courts. But for me, it helps.
Congrats on entering a tournament and good luck, let us know how it goes!