Get the latest iTunes and connect your iPhone
Before proceeding, get the latest version of iTunes if you haven't yet. Having the latest version will give less room for errors. Get a USB cable that can connect your iPhone to your PC. Faulty cables may cause the restoration in recovery mode to fail, so make sure that the cable you're using is working perfectly fine or better yet, use the USB cable that came with your iPhone's original packaging to guarantee full functionality. Connect the USB cable to your PC while leaving your iPhone disconnected.