Just do it!
I bought my first smoker, a Char-Broil Silver Smoker (COS) in 2007 and used it a few times each summer to smoke ribs. I joined this website in 2009 looking for some help with my ribs and began to take backyard bbq very serious. In 2010, I got 2 18.5" Weber Smokey Mountains and a 22.5" Weber Kettle and decided to take my passion for bbq to the next level. It was the knowledgeable and friendly members of this website that convinced me to enter a competiton. So in May 2010 I began practicing heavily in my backyard. After going crazy practicing, making checklists and timelines, making and modifying rub and bbq sauce recipes, I was ready to enter my first competition, The Battle of the BBQ Brethren, in August 2010.
I took 4th place in the pork category and 31 out of 36 overall. I would've done better but I got hammered in the chicken category. (Note: If something tastes good when you cook it in your backyard, don't change the recipe at a competition!) It doesn't really matter what place I came in though, the fact that I had a great time with my father and uncle, met some really cool people, especially the brethren, and learned a few things made it a success.
So, my advice is, if you are considering entering a competition, just do it. Go and have fun and don't take it too serious. Everyone there was a rookie at one point. And you'd be suprised to see how friendly and helpful the other teams can be. If you're serious about it, look in the events section of this website, or on the kcbs website, and look for an event near where you live. Find out if any brethren are competing in this event and pm that person and see if they can mentor you.
Bottom line, if you practice and prepare, and don't go into it thinking you're going to walk away the Grand Champion, you will have fun and you won't regret it.
I had so much fun at my first event that I am now competiting in 2 more events in the next month. If you have any questions, feel free to PM me.
Good Luck!