What is a standard price a property management service charge and what do they do?

Can anyone help rent property for an investor or do you need a special licence.|||generally a property manager harrasses the tenants of a house or apartment about the fine points of the lease, make sure rent is paid on time, and make sure the property isn't being run down.....and i think a rather standard fee is roughly ten percent of monthy rent....and i don't think it requires anything more than a business license, but usually most prop. mgrs. are also real estate agents....hope this helps...