What do you do when you already plugged in your usb cable to upload your pictures from your camera?

It depends on what kind of camera it is. If it's a digital SLR, then you might have to turn on the camera in order for it to connect. Once it's turned on, a window should pop up asking you how you want to transfer your photos and will give you some other options as well. You can transfer them by either just opening up the folder itself and dragging the photos over by hand or most computers have a digital photo transfer program that you can open and select the photos that you want to copy at that time. If this doesn't help, look in your manual and there should be instructions there.