What causes squealing noise coming from right wheel when driving over 30 mph and then grinding upon braking a 1999 Sentra?

That will be something to do with your brakes on that wheel. You may have a stone wedged between the pad and the disc, or your pads are completely worn. Take the car to a brake specialist. If you are confident in removing the wheel safely, inspect the brake assembly once the wheel is removed. You should see a smooth disc and a pad/caliper assembly 'clamped' around the disc, like a hand holding a frizbee. Inspect the pad for anything obviously wrong.