As soon as you get your child's school place confirmed, start talking about it. Tell them the name of the school, the teachers and anything else you know so they can start to process this big life change.
The primary school will give you information about the first day and what to expect, (if they don't, then ask). This can help your child understand what is going to happen. Be realistic though. Children aren't daft. Explain that it is ok to feel nervous or sad and they will get tired, but they can always tell their teacher.