Are you trying to change the resolution of your monitor or get a new video card?
What is the brand and model of your CPU, video card, and monitor.
If considering a video card upgrade, what is the make and model of your power supply (PSU)?
You probably will have to look inside the case to get information about the PSU and video card.
If you are not familiar with computer components, take normal power and elecrostatic discharge prevention safety precautions. If you need more info about this, see the information presented on this web page about installing a video card: