[GTALUG] How to cut out borders of image?

Not really Linux related except for the fact that GIMP in Linux. I have an image that is too large. I don't want to reduce the whole image. I just want to cut out left and right borders, like you would do with scissors. How do you do that with GIMP (I assume it would be GIMP thing). In Windows, I just move the left/right edges with mouse. Searching online for GIMP answers, I get things like transparent layer, selection layer, etc. :-( --William