I raced Cat 5 criteriums for the first time last year. Every race was different. One race ended in a bunch sprint with 2 or 3 guys doing all the work. Another time there was some MTB crossover just getting his Cat 5 races in. He broke on about the first lap and beat the field by a mile.
In your first Cat 5 race you should focus on staying upright, especially if you've never ridden at speed in tight quarters. Stay in the front, if you can. If you're timid and stay at the back you risk being dropped. At every corner the pack strings out like a slinky. Those in the back have to slow more than those at the front, so they have to work harder to catch up after the corner. Eventually it takes its toll and you get dropped.
Staying at the front can be nervewracking for the novice. Speeds are faster, racing is tighter, racers are faster and less forgiving to the inexperienced. Hit your brakes and you'll lose several places and find yourself sprinting to get back on.