Find out applications using a lot of bandwidth, I have an iMac (early 2009) with El Capitan loaded -- 10.11.6. Immediately upon start up, without starting a single program, something in the system starts producing wifi traffic. A lot of it. A program I got from the App Store called Bandwidth+ indicates the rate is between 400 to 500 kbps. Its eating up the bandwidth to the point its preventing others on the same wifi network from accessing the Internet. I haven't found any kind of Process Manager in the System Preferences.

Is there a way to detect the process that is causing the traffic, something that might be running in the background, so I can terminate it?
Use the Activity Monitor program to view the CPU, Memory, Energy, Disk, and Network activity associated with running processes.  It is in the /Applications/Utilities folder.