You can't choose your child's secondary school, but you can let the Admissions Service know which schools you prefer. You should put down three choices of schools, in order of preference, with number 1 as your most preferred school.
It is important that you look around and consider several schools in your area in case you don't get your first choice. Make sure you are happy for your child to attend your second and third choices of school. Only putting down one school will limit the chances of your child getting a place you and they want.