Actually, there are classes. On your entry form you will have to decide which ones you are going to ride. Some shows organize classes strictly by level and test (ie Training Level Test 1 (Jr/AA/Open combined), Training Level Test 2 (Jr/AA/Open combined)), where as others divide classes by division. Still others offer "TOC" or "Test of Choice" classes, sometimes for each level and sometimes for any USDF test. At the shows I organize I offer classes for each division (Jr/AA/Open) and offer a mixture of fixed test and TOC classes. If you need help figuring out what to enter, I can help you with the form. Generally, at first level and below, you can get away with 2 classes per day and above that, stick to one.
Depending on the show, there will be ride times, or there could be an order of go, refer to the prizelist, or ask the show organizer about this. If it is by ride times, tiny is correct, however if it is by order of go you will have to see how long each rider is taking and do the math yourself. As a guideline, each test should take 5-7 minutes.
There is usually not a gatekeeper/whipper in at small shows. In fact at some of the bigger ones there isn't one either!! You are responsible for knowing who you ride after and being there when the bell rings. If you are late and the bell rings, you have 45 seconds to get down centerline.
For the once around, depending on the set up of the arena you may be allowed inside the ring if there is no room to go around on the outside of the ring.
For test callers, I highly recommend that everyone memorize their tests and have a caller only as a back up. Memorize and practice your test before the show. There are far more really bad callers than decent ones.
As far as for judging, I have had quite a few PMs on this, yes the horse is generally what is judged. A judge giving rider comments, docking down for diagonals, etc.. is generally an untrained one. If the horse is lacking balance and you get an "unbalanced" comment on the test, we can all infer that it was rider error. At the higher levels, rider errors are docked down more blatantly (ie, counting errors in tempis), but it is, in general the performance of the horse that is scored. However, we all know that the horse's performance is very, very highly affected by the rider. The only thing not related to performance that is judged that may be considered rider error is accuracy, so ride to your letters!!
(And actually, purposely posting on the wrong diagonal can be a great strengthening exercise)
As far as for other things to know. I don't know where you are from, but generally attire rules expect the horse to be braided and well groomed and for you to be turned out in light breeches and a show jacket with your hair neatly tucked in your helmet, or a bun.
As far as the lowest levels, there is intro, which is walk and trot and training level which is walk/trot/canter. There are no restrictions in these classes due to age, experience, etc.. so feel free to enter them!
For more information, check out the USDF website (if you are in the US) and feel free to email the show organizer! As a show organizer myself, I way prefer emails weeks in advance with questions over incomplete entry forms and tons of questions on the already busy day of the show. You can also PM me if you have anymore questions.
They say money doesn't buy happiness -- well happiness doesn't buy horses!