i turn on my tower and it turns on fine but when i turn on my screen the light is green for a few sec. then it turns yellow how can i fix this

Try turning on the monitor BEFORE you start the computer. See if that works.

Try wiggling the data cable where it goes into the monitor. If the colors vary then the internal cable wires are frayed and it needs to be replaced. Make sure it's connected good at the video card also. If it's neither of those then likely the monitor is bad. Try another one if possible. The other possibility is the video card but it's least likely.

wheres the video card located so i may check

The cable going from the monitor to the computer connects to the video card. So wherever it connect at is the video card. You may have an on-board video adapter, that is the video is on the motherboard and not a separate card. So wherever it connects just make sure it's not hanging loose.

There is no need to even turn off the monitor. When you shut down the PC it turns the monitor off. Also there is no need to even turn off your PC just reboot it evey day or two. Jesus Loves You! Keyboard not detected. Hit F1 to Continue. BREAKFAST.SYS HALTED Cerial port not Responding!!

ChrisG Where do you get the idea that you don't need to shut off a monitor? If it is turned on it is still pulling AC current weather or not there is a video signal going to it or not.