option 1, emulator play games on your computer and add them as a video source.
option 2, get a real time video capture card/device to use with your computer. (HopCentury capture box, USB3HDCAP, Razer Ripsaw, Elgato HD60 s/pro, roxio Game Catpure)
The capture device needs to have inputs that match your gaming device's outputs. You'll need a peace of software to combine all your video streams (web cam, console). Depending on your streaming platform that might be some thing like manycam or OBS. Your computer is going to need to be strong enough to handle the input of both video streams into your computer.
Also check the TOS of the website your streaming to. Some prohibit streaming games and other copyrighted content.