fuel gauge is always on empty

For as long as I've had my car, the gas Gauge has never worked. I have been forced to use my trip meter to know when to refuel, but there have been several times I ran out in the city or on the side of the road.

I tried changing the fuel level thing in the gas tank (or at least my dad did), but my parts car was a slightly different model and wouldn't work (even though it's the same year)