I have been riding since I could sit int he saddle on my own. Even before that, I was lead around on my horse with someone holding me on. My first show was a western pleasure class when I was four, just walk/trot. When I was five I did walk/trot/canter, and when I was six I started barrel racing, then around eight I was reining, and by ten I did all the cow work. I specifically started jumping when I was twelve, but reining or reining cowhorse has always been my favorite thing.
My Mom taught me a lot about the basics, but at the critical learning point she was still not capable of teaching me advanced techniques fluently (She's a natural, but not a good teacher) so I took lessons at my family's trainer that trained my Mom when she was my age, and is now teaching me. As I progressed, she started putting me on her horses to show. This is the more recent thing. I've really only been competitive with her
horses since I was thirteen, and I'm nearly sixteen now.
I do preach teaching lessons, but there's something I preach even more: Comfort. Take your horse out on your property or on a trail ride and just learn their quirks. My trainer taught me so much, but in the end it was my big ol' sorrel horse that really showed me the ropes, and in ways I taught myself a lot of things about dealing with differant situations.
As far as ability goes, there really isn't a good way to tell in the beginning. If you just want to compete for fun, then try everyhing. No reason why not. And remember, sometimes you have natural ability for something but you don't want to do it. I knew this girl who was the best english rider I had ever seen, but she didn't like doing it so she started reining and she sucked. Badly. So she worked really hard and eventually got better and better, and now she kicks some serious butt at every show.
It's up to you in the end, really. :) :) :)