Most schools will have a written explanation of what they expect from their prefects, either as a school document or on the school website, so you should look at the one for your school to get exact information.
Generally, prefects are given different responsibilties depending on their role. For example, there are corridor prefects who walk around during breaks, make sure everyone is OK and keep an eye on behaviour. There are class prefects who may help with school work or helping the class to run smoothly, and senior prefects can help with running the school including sports events and parents events.
Depending on the type of school you can also have house prefects (some of the big public schools are divided into 'houses' or departments) and in most schools there is a head boy or girl or both, who organise and help the other prefects.
You can see an example of one school's prefect policy here.