How do i get back into athletics?

when i was about 12 i used to do athletics..id run on the track during the summer and id do cross country during the winter..i was pretty good at it too..i started when i was 8 and gave up when i was 13..im 17 now and i really want to get back into..ive lost all of my fitness so how can i build it up again as fast as i can?? and also should i go on a special diet??what do you recomend eating and not eating??how often should i train and how long should i run for my age??im a girl by the way..thanks in advance..:)x