NOT ALL JUMPER CLASSES ARE JUDGED THE SAME!!
The first thing you need to do is figure out what "table" each classes are. The three most common I see at schooling shows are...
Table II, Section 2(b), which is also known as "timed first jump off". I think these are the most common classes. You do your first round and then STOP if you're clear (or leave the ring of you've had faults). Wait for the buzzer and then procede to your jump off. You have 45 seconds from when the buzzer goes off to get to the first jump off fence. You ribbon according to the least number of faults, and then speed. So if two people go clear in the jump of, the winner will be who is the fastest.
Table II, Section 2(c) -Also very popular. It's also known as "power and speed". Somewhere in your course is a designated finish line. You begin your course (power phase) and if you are clear you -without stopping- begin the speed phase. So it's like one giant course but you start off more carefully and somewhere in the middle you should pick up the pace a bit. If you are not clear in your power phase they buzz you out and you leave the ring without doing the speed phase.
Table II, Section 2(a) -aka 'timed first round'. All you do is your first round, and there is no jump off unless there is a tie for first.
Last week at a show I did 3 jumper classes. The first and third ones were timed first jump off, second class was power and speed. The week before I did 3 jumper classes and each of them were the 3 tables I mentioned above.
As far as clothing goes, I'm generally a stickler for looking your best (my hunter background!) but at schooling shows I wear a nice polo (tucked in).
Define "too fast". There is actually a table that's "optimum speed" (I've only seen it at rated shows). You get faults for going UNDER the time given! But generally if you go "too fast" it means you were so fast it made you sloppy. I personally hate it when people run so fast and crazy that the audience gasps. Sure, you can get away with it when the jumps are small but when they get bigger you need to get your turns tight and get the "technical" details correctly.