If you're on the PS4, just press the share button. You'll have the choice to record or save the screenshot. Then you can go into the gallery afterwards, save it to a USB drive then upload it to a computer. You may be able to upload it to Facebook or something along those lines too, if that's easier for you.
Edit: Just saw you were on PS3, my bad. Hit the home button, go to photos and underneath that, there should be an option to screenshot.