You can apply for Year 11 and then probably do some GCSEs but not all of them. If you have just turned 15 join Year 10; 1996/1997 are the birth years in which year 10 students are in.
As for options it depends on the school; normally you get 3 subject options along with the core subjects (English, Maths, Science). However some schools do the English Baccalaureate which is English, Maths, Science along with a language (Depends on what the school has to offer), a humanities (History and Geography) and then a subject.