Start out by eating more food, especially protein.
Aim for around 1 gram of protein per pound of bodyweight per day.
Since you are 11 stone, that is 154lbs, so you want to eat around 154 grams of protein per day.
You can get protein from many sources including meat, poultry, fish, eggs, milk, cottage cheese, yogurt, peanut butter, soy (tofu), etc.
You can also get protein powder, which is typically made of whey.
Lots of good nutrition advice can be found here:
Nutrition For Newbies & Must Read Threads - Bodybuilding.com Forums
Do home bodyweight exercises like push ups, pull ups and sit ups.
You can purchase a relatively cheap pull up bar that hangs inside a doorway.
Next, get a set of weights and a bench, or join a gym and use their weights.
Craigslist is a good place to look for a cheap set of used weights and a used bench.
You will need to lift weights to put on muscle.
Situps will help you tone your mid section (abs).
Be patient and do your exercises every day.
Muscle doesn't happen overnight. It can take weeks and months before you see noticeable growth.
I recommend reading some of these Teen Transformations on bodybuilding.com:
Bodybuilding.com - Teen Transformation Articles!
They tell you both their diet and exercise routines which helped them transform their bodies.