Work out which schools you are interested in now. How many preferences you get varies - in London parents get to put down 6 schools. In other parts of the country you get 3 or 4.
Many schools have Summer fetes and fairs. These are an opportunity for parents to get a limited look around and a feel for the school.
Keep in mind you are letting the local authority know your preferences - you are not actually choosing a school.
The local authority gives priority to applications from parents that are made on time when allocating Reception places. So it is important to get your application in :)
The school won't know where you list them in your application. They only know that you have applied.
The local authority sends the list of applications to the school, and it will put all applications in order according to how well the applications meet the school's admissions.
If you meet the admissions criteria for more than one school, the local authority will offer you your highest preferred school, and disregard the other offers - you will never know about them.
If you do not qualify for any of the schools on your list the local authority will allocate the nearest vacant school place to you AFTER all the preferred applications have been offered. This means it is likely to be an Undersubscribed no one wants.
It is advisable to always make sure that at least one option on your application is a school you are really sure you will get into even if it's one you aren't that keen on (but it's better than a school you don't want that's two buses away for instance).