OK, breathe ...
The fuel gauge and associated lamps etc. have two seperate circuits. The low fuel lamp is fed from the ignition (behind the instruments, white wire) and then a white / slate wire goes from the other side of the bulb to the sender unit. The bulb will only come on if you earth the white / slate wire somewhere. There is a seperate copper sector in the sender unit that does this when the level falls too low.
The second circuit is the fuel gauge, which is fed by a green wire (behind the instruments) and the other connection goes via a green / black wire to another terminal on the sender unit. The higher the resistance of the sender unit the lower the gauge reads.
Both circuits then go to earth via the other (black) wire.
EDIT:
Try unplugging the green / black and white / slate wires form the tank, turning the ign on and touching each one to ground in turn - the low fuel lamp should flash on and off and the fuel gauge should go to full and back respectively.
EDIT2:
You have checked to make sure the sender arm has a float on it and that this float can go up and down, haven't you??