It's tough to explain to a rookie why to just compete in the sanctioned portion of the cookoff. I'll tell you, for many competitions you can piece-meal the entry fees and only compete in the category that you want to compete (thereby keeping costs down). Historically the judging is the single biggest factor, and that the mark-to-hit has largely been defined. In unsanctioned competitions (back-yard if you will) the mark to hit usually isn't defined and judging is very willy-nilly. If you're wanting to compete to learn and to refine your barbecue, competing in unsanctioned competitions largely will not do this. I'm of the opinion that you're better off starting out competing in the sanctioned portion/comps, even if it's just 1 or 2 categories. You'll learn about hitting HARD enforced turn-in times, hard enforced rules about garnish or not, etc.
Also, something to know early on, not every BBQ competition is the same, not even sanctioned competitions, as there are many varied sanctioning bodies. FYI: they can and do define what is the perfect piece of BBQ differently. Learn the sanctioning body; learn the rules; then compete. And, HAVE FUN. Ultimately it's just about barbecuing and producing great tasting tender meat.
Lastly, know that it IS a competition. I've seen more than a few rookies come to compete thinking that because their grilling evening went well a few months ago that they're ready for competition, only to find out that 95% of the cooks at these things REALLY REALLY know what they're doing. PRACTICE, PRACTICE, and when you need a break, PRACTICE. IMHO a competition is no place to try a new technique or recipe. When trying recipes at home, taste your barbecue stone cold sober. Be like a judge will be. Trust me, food that tastes great after you've had boilermakers for 6 hours doesn't taste so great when you're stone cold sober at 10am having had only coffee and orange juice to drink all morning... Also, get multiple peoples opinion (when they're sober as well), and if you can, compare recipes against one another. Meaning, when hungry, most anything tastes great. Get them to try them side-by-side. Then you'll get an honest comparison and know how you're progressing.
Best of luck!!!