Right- well generally speaking...

The comp organisers should send you the running order and tell you what class you are in etc prior to the comp.This comes on a little card which you'll need to keep and bring along with you (if at an agility aid organised show). They should also tell you where to park and send directions to the venue.

Generally speaking it's a good idea to get there early- find a good parking space in the shade where is nice and cool so that you can leave your dog in the car for a moment if necesary with some water etc...
I like to get there at the start of the show and watch some other classes and the handlers, their handling techniques etc...
Then walk the dog around and get them used to the environment, give them a chance to stretch their legs. Sometimes there will be an exercise area for the dogs or somewhere nearby where you can walk them if necessary.

When it is time to walk the course, get a friend to hold your dog or pop him/her in the car for a moment, whilst you walk the course and work out where you need to be, how you need to direct the dog etc... It is advisable that you walk the course at least 3 times. You could even get a friend from your agility club to walk it with you and give you some handling tips.
When they announce your class, you need to have your dog ready on a lead with you, bring your card and join the queue or go up to the ring steward. No toys or treats are allowed in the ring. Although many have a toy attached to the end of their lead so that they can play tuggy with the dog after their round. Present your card to the steward and tell them what number you are. When it is your turn the steward will direct you on and give you a bucket to put your dog's lead in. Some people prefer to walk their dog up to the start offlead to get them focussed whilst others prefer to set the dog up at the start and chuck the lead in the bucket then. Totally up to you and what you feel comfortable with.
The steward or judge will then say "ok *name* when you are ready" or simular and you run the course.
Once you have completed the course you need to collect your dogs lead etc...

Rules of the showground:
Obviously these vary from competition to competition but stuff like:

Picking up dog poo/litter
Not disturbing others/ any livestock
No harsh handling methods

One thing some competitions say is to not play with squeaky toys by the ring has this can distract the dog that is working in that ring.

Yes you can walk around the show ground with your dog. It's probably advisable to do so onlead though.

Facilities- most have portable toilets (the types you see at festivals), burger/ food vans/stalls and lots of doggy stalls, selling human agility equipment (fleeces/agility shoes etc...), agility equipment (jumps,tunnels etc), dog leads, collars, toys, coats etc... Sometimes local rescues may have stalls to fundraise.

Hope I've covered everything you asked

__________________