i wouldn't recommend the pedometer, most skinny people just dont have one. as long as you try to walk everywhere, starting with 20minutes a day and building up to an hour or even 2 or 3 if you have time then you don't need to count the steps. the trick is to find activities that you enjoy.
listen, when i was 15-16 i had eating issues. i am about your height and i weighed over 11stone, which i felt was too much and i developped a very bad attitude to food becasue i was either "watching my weight" or "not watching my weight" which meant stuffing myself. with unhealthy food
i am now in my twenties, weigh just over 9 and a half stone and i think i have developped a healthy relationship with food.
i have got really into cooking with fresh ingredients. proper food - vegetables, wholegrains, fruit and veg, fish, lean meat- makes you feel good and it makes you look good - not only body but it makes your skin and nails and hair nice too. the thing about learning to cook is that you start seeing it for what it is - something that gives you nutrients.
but also don't deny yourself of some chocolate if you fancy some, you don't have to deny yourself to be thin. but instead of a mars bar, eat a small green and blacks. if you like dark chocolate then this is really good for you.
but most of all, don't worry, you are still young so you have time to learn to eat healthily. lots of people that were skinny at your age never get this kind of education and when you get to my age, they start to put on weight and don't know what to do. that is when you will have the advantage if you educate yourslef now about these things.