1) You should be applying to sixth forms now. The deadlines can vary but some are in November, December and most are by February - college/sixth form is different so you need to check this.
2) When you send your application off (hopefully before the deadline), they review all applications and some, if not all will invite people they like to interviews.
3) You then get offers (in most cases).
4) On results day, you get your results and you decide which sixth form you want to go to from the ones you have offers from - presuming you get the grades.
5) you then go to the enrolment of your chosen sixth form and bring the documents they require e.g. ID and results slip.