To me, the basic rules of outdoor racing would be:
1) helmets mandatory, other padding optional
2) first wheel to the finish line wins
3) stay on the course
4) don't trip, punch, or physically impede your competitors
After that, it all depends on what kind of race you're in, as small local races are a very different experience than big, national ones.
Forgive me if I state things you already know -- I didn't know any of this when I went to my first race.
When you ask about rules, I also figure you're asking about categories, because that's usually what defines the common practices in outdoor skating.
Because this is a pack sport -- like bike racing -- you register for a spot in a certain category, which then allows you to skate with (that is, draft in a paceline with) a specific group and no other.
The usual categories are Rec/Fitness, Advanced, and Pro. They all have separate starting times, with the Pro category further divided by gender and age (men and women, Elite and Masters).
However, if it's a small, local race or a long, hilly course, there may be a mass start, which means all categories start together. In a mass start, you are free to skate with anyone at any time, regardless of age, category, gender, registered distance, etc., and the categories are mostly there to sort people by age and skill level for their finishing placement.
On a practical level, the categories are less about raw speed than levels of pack skill and competitive attitude. If it's your first race and the categories have separate starts, choose Rec/Fitness -- people will be very friendly, you can wear any kind of skates, and you don't have to skate in a paceline unless you want to. In Advanced, where people wear racing skates, competition is somewhat friendly, but you should already know how to behave in a paceline or it won't be fun. In Pro, competitors are openly hostile (to strangers, at least), and competing there requires a host of dependent skills you can only get through lots of racing experience. Even if you feel like you're pretty fast, you will not enjoy a race if you try to compete in a category above your skill level.
Did you have a more specific concern about rules, tactics, expectations, etc.? You can also write me privately.