The first place to start is your child’s current school. If your child is currently attending a prep school, then that school should be liaising with you to provide advice and guidance as to which senior school(s) you should be considering. There are also many publications available to this end, although do be sure to read around any subject. Websites like Mumsnet can be fantastic for giving very frank advice on school entrance, but like anything a lot of this will come down to personal preference.
By combining all of these sources of information together with your own requirements you want to try and make a list of four or five schools you want to consider. The temptation here is to pore over league tables and rankings. These do have their merits, and in some ways are a very effective metric of how well a school is performing. That being said, beyond a certain point (say a 99% vs a 97% passrate at GCSE) these can feel like splitting hairs. Also, do be aware that many schools don’t publish results and so won’t be listed in these tables.
Consider making a list of essential criteria you want a school to meet. At this point, sign up for their respective open evenings, and go along to get a feel for the place. They will be very keen to show off various facilities. Do try and involve your child in this process as much as possible; after all, it is going to be their education! Show them prospectuses, take them on visits and ask them what they would want from a new school.