You apply for all the schools in one form (online), via the council.
Some schools, often including faith schools, ask you to submit an additional form - a SIF or Supplementary Information Form - to the school.
Admission to the school can only be according to that school's published admissions criteria. Most primaries, for example, admit according to distance - how far you live from the school. Some faith schools admit according to attendance at church or mass. They will also have other priority categories: SEN / children in Care / siblings.
The LA sends your application to all the schools you apply to, and those schools assess your application against their admissions criteria. They tell the LA whether or not they can offer you a place. If more than one school on your list says 'yes' the LA will offer you whichever of those schools is highest up your list of preferences. You only get one offer - the LA will turn down your lower choice places and they will go back into the computer to be offered to others who applied.
January 15th is the national deadline for applications. So in the summer or autumn before lookout for Open Days at the schools near you. Look on their websites or in the LA Booklet about school admissions to check the school's admissions criteria.
Check MN for discussion about admissions before you put in your application, especially to understand the system and make sure you choose a school that you do have a good chance of getting into.