Since I have onboard ethernet to my motherboard, do I need a network adaptor?

If you have a 2nd pc then you might buy a wireless adapter to connect to your router especially if your home isn't wired for ethernet. But if you only have a broadband modem, you wire to the LAN. Or if you have a modem and router for phones/tablets you can still have your pc wired to one of the 4 ports on most routers.