I was a gymnast for most of my childhood, and I started weight training in a gym when I was 15 years old. You are definitely not too young to start, but you do need to learn the basics first.
Like Deicide suggested, learning how to do basic exercises and eating healthy foods, and sticking to a consistent diet are all good ways to start. It depends on your current weight and body fat composition too, in terms of how much food you would want to be taking in. But starting with healthy eating and trying to eat several small meals a day is a good way to start.
If you join a gym, you may want to consider hiring a personal trainer from the gym to at least show you the ropes...how to use the machines, what exercises work for which muscle groups, how to put together good workouts, etc. If you worked with a trainer for 3-4 workouts, you can get a general idea of how to train in the gym. There's alot of learning that you can do on your own too if you are really interested in making this part of your lifestyle now and into the future.
Check your PMs.
