Road Bike Racing - Question on MB Race Catagories

I have been riding bikes for 10 years but I know nothing about racing. I went to the Grand Prix in MB this weekend as a spectator and thought it looked awsome, great event.
I know there were many catagories and I wanted some information on whethere there is a catagory for an "average joe" who has never raced before.
I watched the Pro, Womens Pro, 45 plus and 35 plus and i know for a fact that none of the catagories would be for me.
I was told that Cat 4 or Cat 5 was for beginner racers but since I didnt see them race I really cant judge if I should or could be in that class. Could someone explain in layman terms what type of rider enters that class ?
I ride my road bike about 100 miles a week. I am not a sprinter but i ride the hills in PV all the time, usually only about 20 miles at a time though.

No matter how good someone is or isn't, they all start as Cat 5s.

Then, after they have 10 mass starts (i.e. not a TT), they upgrade to the 4s.

After that, go to page 5 here: http://www.usacycling.org/forms/USAC_rulebook.pdf

Cat5 is where you'll start. You'll need 10 starts there to move on to Cat4, which is for more "experienced beginners." This is typically the level where folks start to decide whether they want to quit or keep going. It can be difficult to move up from 4, and you will more than likely lose some skin/spill some blood at some point. It seems like half the guys in the 4's take it very seriously and work very hard to move on, but a lot of them are just there, still trying to figure it out.

since you're in the l.a. area, find some training rides before you start racing. the group experience will teach the the pace of racing and give you some much needed pack handling skills. in you area you've got el dorado park on tues nights and the long beach crit on tues and thurs, i believe. check out socalcycling.com for their list of training rides