It’s very difficult for me to control my dreams. Over the years i have found that I not so much control them, but I am aware in them. Not aware that I am dreaming, so much (sometimes that doesn’t change the fact that it’s terrifying!). But I make conscious decisions and choices in my dream based on what my dream self knows to be true about the environment. And once you start doing that, you can begin making things up that you “know” to be true. Or, start playing LOTS of first person shooter video games until you can pause and restart the checkpoint in your dream, as I do. Or, just ride it out. I hate those awkward linear dreams that only come once every long while. As you get older, your nightmares should subside (I am 21 and have noticed over the years they get easier to handle)…well, unless you’re older than me, in which case, I don’t know what to tell you!
There are many ways to induce lucid dreams, but since you have nightmares, you ironically have a good start to induce lucid dreams. The technique you should go for is called reality checking. Reality checking is used to determine whether you are awake or dreaming, and is often a physical check, commonly done by pinching your nose and trying to breathe through it. If you can, you are dreaming. Another common way to do a reality check is to try to press a finger through your palm.
During nightmares, you will be scared. This is when the reality checking comes in. A technique that would be useful for you, is to perform a reality check whenever you are scared.
And as you already know, once you are lucid in a nightmare, you can control it and make it into a good lucid dream. This is actually considered the most effective way to get over nightmares. When it comes to what to do once you are lucid in a nightmare, I suggest you read this excerpt from the book “Exploring the World of Lucid Dreaming” by Stephen LaBerge: http://www.lucidity.com/EWLD10.html as it explains well how to use lucid dreaming to overcome nightmares.