Being 39 you would be in the Men's 3 division. Men's 2 goes from 25-34, Men's 3 from 35-44 and Men's 4 from 45-52 The max speed is 34mph. In AWSA/USAWS tournaments the minimum speed is 15.5 for all divisions.
How a tournament running order is constructed is up to the event organizer and I have seen many different ways to do it. Some seed within divisions, some seed everyone based on score, some do completely random order, some do some other sort of grouping based on drivers/judges/scorers needed. Typically the running order is posted before the event so you know when you will ski. The best way to figure this out is to contact the tournament director as each tournament could be different. -- Edited: The scores used to seed are typically pulled from the USA waterski ranking list which can be found at usawaterski.org. If you don't have have any scores you may be seeded as a zero score or the event organizer may ask you for your typical practice score and seed you that way.
AWSA is a grouping within USAWS so you just need to be a member of USAWS and you would select AWSA as your discipline or sport group.
As far as starting speed/line length I have seen it all over the board everything from 15.5 for W1 skiers on trick skis, 28off @ 30mph (for a M4 skier), 32off 34mph for a M2 skier going 36mph. Typical though is 15Off if less than max speed. If at max speed then whatever you are comfortable with. The idea is to select a line length and speed that you are comfortable running every time out no matter the conditions. I think most people tend to pick a line length that is 3-4 passes below their expected average score. So if you normally get into 32 you might start at 15off so that you would be skiing your 32 off as your 4th pass.
The other thing I can suggest is that tournaments can become very boring if you are just waiting around for your next time to ski. In the mean time ask questions of the officials and other skiers, as how you can help and become involved. It helps pass the time while you are waiting between rounds.