I just added a new graphics card and it came with two other ports to plug a monitor in. I was wondering if I would be able to use dual monitors because of this, if not could I just plug it into my onboard card and would it still work?
That's how it works. I'm not aware of any card out in the past 5 years that does not support dual-monitor. Simply plug the monitor in and turn it on. Afterwords configure whatever you will in the driver's settings.