Help me make a service agreement for on-site computer services?

Hello everyone, I'm new to the forum. I am starting a small business where I go to peoples' homes and help them with their computers, same kind of stuff geek squad does. I have a website and everything, but I'm not sure what to do for a service agreement, or if I need one at all. I am planning on requiring payment as soon as I'm done, before I leave their house. Any advice would be greatly appreciated.