My experience with property management co's are that they are expensive. If you want a hands off approach on maintaining your investment property then a property management company is the way to go. Depending on the company ,they will either charge you the equivalent of one months rent to set up and then a monthly fee of anywhere between 10-20% of your monthly rent as the fee. Some I have seen charge a start up cost of 25% of the first months rent they all vary.
One thing that you will need to put in the back of your mind is that your sweet home is turning into an investment property and the future tennants may or may not take care of the place like you would. Be prepared to do a little fixing up due to wear and tare that the renters do to the place.
I would also first check with a local Realtor in your area and ask them who they would recommend. Some Realtors will manage the place for you as well.
Good luck in NJ!