my 98 chevy cavalier has not been driven since 2005 and has sat with empty gas tank..do I have to install new gas tank?
The reason for filling the tank when storing a car is to provide minimal space for moisture to collect and rust the metal of the tank from the inside. It's not a guarantee, but the empty tank may be harmless. If you feel up to it, you could try to remove either the pump or sender (or one unit) if your car allows it and look inside the tank. Provided there's no serious rust inside, it should be fine to continue using it.
There's also a school of thought that gasoline ages and turns to a varnish-like compound with time plus air, so if the tank was empty and the lines have dried, you're probably in a better state than someone who left the car in ready-to-start condition for a decade.
If you're concerned about the state of the tank, add a small amount of gasoline and check for leaks, or the smell of fuel around it. Chances are, you're probably fine.