Do you have to pay agents to help you with publishing books?

I've been researching how to get books published for a little bit. I've learned a lot and I've heard a lot of people suggesting getting an agent to help you with contacting publishers. I've also heard that publishing shouldn't cost you anything. But does that mean just the publishing process or does that include everything you might do during the process? What I'm mainly trying to ask is do agents bill you for their services? I almost think they would because they need to make money too... Any help clarifying this for me would be greatly appreciated.