They go to school the September following their 4th birthday. You can apply from September the year before usually, up until January.
It depends on your local councils process. They start school the school year they turn 5 so you have to apply the school year they turn 4. The timing will depend on the council. Some open applications September some don’t open until January. Generally you will be notified by the council when to apply but it will also be on their website (my local council doesn’t open primary applications until November but high school are already open)
You don’t need to worry yet. The nursery will let you know when it’s your year to apply. My eldest is born in June, so he turned 4 literally 3 months before starting school. Applications open in the September, and you have until the January to submit your preferences (in my area it’s 3) - it’s a good idea to see what schools are in your catchment area and in the October/November they normally offer school tours so you can go and get a feel of the school and see if it’ll suit your child.
You can apply from October
It will be the January before they start however do you want your child to go to a school nursery? Your local council website will give more details but also I imagine your nursery may give reminders to parents.