This is my third season of 'cross racing, so let me see if I can answer some of these questions...
1. I have never seen a citizen category. It used to be A, B, and C's with various Master's brackets. Now with USCF categories, it's Cat 4 and up. So you would be a Cat 4. You can get a one day license to try it out. You can get an annual license at most races or at the USA Cycling website. The license will expire at the end of the year.
2. Cat 4 races are 30 or 40 minutes, which usually comes down to 3-5 laps depending on the course.
3. There are tons of web resources with video's and the like. Google is your friend, as is youtube.
4. A CX race is like a combination of a time trial and a crit. If your doing it right, you should have your HR pegged and want to vomit. Figure on a killer sprint at the start as everyone is tries to get the hole shot. Then it's a hard sustained effort for the rest of the race while tackling technical sections. The last lap could be pretty fast. To train for this, practice 0-max sprints, do 20 min intervals, and your transitions. Take your bike off-road and ride some single track to get a feel for traction and the like. Finally, walk into your first race thinking of it as a learning session. I know Cat 3 road racers and Sport MTB racers who got shelled in their first CX races.