Forgot to add - I went from beginner to Expert classification in less than one year, and had a great time doing it. Your first match you are a mess, then you start to really figure things out and have lots of extra time. Service rifle shooters tend to be really helpful with beginners and WANT new people in their sport. By the time you get to your 3rd or 4th match, you will definitely have the hang of it.
I agree with tangeant on the ammo. Heavier match ammo will help you, and it is not really all that expensive in .223 (as opposed to .308 or .30-06).
Eventually you will want to get a true Service Rifle competition upper so you can shoot the 80-grain loads. A good one for you might be the Armalite upper for $675. Sure, there are more expensive, possibly better uppers out there, but the Armalite is a very viable choice. It got me up to Expert so far, so I can't complain.
You will have fun, become a much better shooter, and feel a real sense of accomplishment in competing against yourself and improving. Can't beat that!