Tupperware makes a gripper that you wrap around the lid and this works great for opening most lids. When that is too tough, running it under hot water or lightly tapping it on the counter works.
Just take a fairly sharp kitchen knife and place and hold the point down on top of the lid with one hand. Take your other hand and it the butt of the knife downward. It will make a small slice in the lid. It will unscrew easily.