Don't be scared of the racer lingo. If you are a man you will be in the Category 5 (or mixed 4/5) race. Women start as Category 4 racers. This is just the name for the beginner categories. (You move up in categories based on specific criteria. But don't worry about that for your first race. Just go have fun.) There are also "master's" races based on age at many events.
The Idaho race calendar is a good place to start and find a race. There are links to race flyers that give
details as to location, time, cost, categories, race distances, etc. Many of the clubs maintain a copy of the calendar on their website.
It is good to have some
group riding experience before you attempt your first mass-start race.
Many of the local bike clubs offer group rides where you can meet some other racers and develop basic group riding skills. You can get some basic initial tips on riding in a group here and some here as well. Here is an entire Website devoted to New Bike Racers
(A hat tip to our friends at the Minnesota Cycling Federation.)