My fuel gauge keeps going up and down! what does that mean????

You can either purchase or most places borrow a fuel pressure gauge from your local part store. You should have a schrader valve on the fuel rail where you hook the gauge. You should have over 40# of pressure at the rail when cranking the engine.

Could be power fluctuations to the fuel gauge sending unit, a bad sending unit or a bad gauge. The sending unit would be the most likely cause.

The fuel sensing unit is going bad. It will have to be replaced for accurate measurements. Or you can just fill it up and keep track of the gallons of gas you use (from the mileage you drive). Replacing the fuel sensing probe is expensive because they have to drop the fuel tank.

The fuel sending unit is bad. When they go bad they will read at different levels than what is in the tank. It could be grounding out but not likely when you remove the power wires.

Cheap air/fuel gauges simply tap off the stock oxygen sensor. That sensor can only tell you lean or rich and the car should idle roughly at 14.7:1 and go rich under acceleration. Cruise down the highway at steady 50 mph and see what the gauge does. Should be calmer.
At idle the needle can bounce around quite a bit. Means nothing. It should however go sometimes rich and sometimes lean. That's how the computer adjusts, if it stays lean too long, the computer tweaks the injectors richer. If it stays rich too long, the computer tweaks it leaner. If the computer cannot adjust the ratio, it will set the check engine light and in that case it should be a flashing check engine (meaning stop the car and tow it to a shop - flash means the catalytic converter can burn out).
If you want to know the actual air-fuel ration you need to buy a wide/band oxygen sensor and related gauge. They are much more expensive and will typically run a few hundred dollars. If the car is stock except for K&N, don't bother with the air/fuel gauge.
As for computer hookup. Go read up about "LT1Edit". Don't buy the software (too expensive for a stock car) but you'll understand what software is out there to monitor your engine (and what hardware you need to buy in order for your laptop to talk to your car).

FUEL GAUGE GOING UP AND DOWN SOME TIMES FUEL MOVES AROUND IN FUEL CAUSE FUEL GAUGE TO GO UP AND DOWN.I HAVE SEEN FUEL LEVEL SENDING UNIT HAS POOR GROUND OR SHORTING OUT.A BAD FUEL PUMP CAUSING POWER SURGE BECAUSE PUMP BEARING BAD THAT WILL CAUSE FUEL GAUGE GO UP AND DOWN.

first check fuel pressure with a mechanical fuel gauge that attaches to fuel line schrader valve. around T.B.I. if no pressure with key on check fuel pump relay for correct operation, and also the oil pressure switch has the fuel circuit to it so possible oil pressure switch also. fuel pump has a check valve to keep fuel from going back to tank, so when u turn key on fuel pressure should go up within 1-2 seconds on gauge. if fuel pressure goes down quickly, possible bad fuel check valve, change fuel pump and fuel pump check valve (located right above fuel pump inside tank)

To test the gauge,unplug the connector at the tank,the fuel gauge will go to full,then ground the wire to the fuel gauge,and the gauge should go to empty,I use a test light for grounding the circuit,to keep from grounding the wrong connector,when doing so,the test light will glow dim,and the gauge will go to empty slow.

This would be 1 possibility, can you hear the fuel pump whine or making a noise when you first turn on the key?
It should whine for a couple of seconds to pressurize the fule system then shut off until you try to start the engine. The best way to check is to connect a fuel pressure gauge to the fuel rail connector (looks like a tire valve stem) and the gauge screws onto this fitting. When you turn the key on you should see the pressure gauge read the pressure going to the fuel rail, when you start the engine you should see the pressure maintain pressure to keep the engine running. If not then the could mean that the fuel pressure is weak and would need replacing. Hope this helps

you got a problem with your fuel gauge sender that works together with the fuel pump inside in the fuel tank, that sender is a wire that could be burned and is sending wrong voltage to the fuel meter on the dash board,replace it as a unit or if you want to save lot of money buy it separate from the fuel pump.