Regardless if you want to specialise in a track or field event you need to be in top physical condition.
Being a professional athlete requires a lot of determination and discipline as you will be doing a routine of physical activity every day.
Diet is very important to a professional athlete so it is very important to take guidance from nutritionists and doctors. You can also do your own research in books or on the internet. As you develop as an athlete you need to be more aware of what you are eating so it means a lot of label reading when you are at the supermarket.
Doing a course in a sports science related subject can help you gain an understanding on how to and how not to train.
At school take advantage of all the facilities and equipment. Try as many track and field events as possible until you find the event you like the most and specialise.
Find out if you have a local athletics club and join. You may even have more specialised groups in the local area like running clubs.
Finding a coach you can work with is very important in your development.
As well as a coach find a training partner to work with. Competing on a daily basis can be very productive.
Athletics equipment and clothing can be expensive but it is important you have the best especially if you are a runner, if you have running shoes which don't offer you the right support it can cause injuries later in life which stops you from reaching your full potential.
As an athlete you need to look after your body which means not over training or staying out until all hours and getting little sleep.
Try to progress as far up the schools athletics system as possible as if you are successful at local school events you can be invited to compete at regional and even national events. Competing at a national youth or schools event can get you noticed by major players in the athletics scene.
Outside of schools you may find that there are local youth or amateur athletic events you can enter.