My 2000 pontiac montana wont start,it just turns over,both oxygen sensors are shot and the egr valve is stuck closed,which one would be the main problem

The EGR will not open at idle or when cold, so if it is stuck closed this will not affect anything. If the O2 sensors are reporting 'too rich' (not enough oxygen) signals, the ECU will respond by leaning to fuel injection cycle to the point that there is not enough fuel to start.

How to check? An issue with oxygen sensors is that they steadily lose effectiveness with age and they can under perform for quite a while before they trigger an error code on the ECU. Ensure the connection to the oxygen sensor is robust and clean. As the oxygen sensor only works when hot (they have their own in-built heaters, hence two wires for heating and two wires for sensor operation) and so there is the danger of getting burnt by working on them so a safe approach is to find the electrical connection on the wiring harness remote from the oxygen sensor and to make voltage measurements there. Most garages have systems that can record the amplitude and frequency of the voltage peaks being produced by the oxygen sensor. A less sophisticated means to get some impression of the oxygen sensor function is to use a moving coil galvanometer type voltmeter (analogue needle on dial). Setting the voltage range to 1 volt and by attaching the meter leads across the sensor wires it should be possible to see the rhythmic pulsing and the voltage range of the operating sensor output. If no pulses are seen it could be either a break in the wire or a fault with the sensor itself.

Your vehicle is about 10 years old and most oxygen sensors last about 10 years or 100,000 miles....check to see how easy they are to get at and what the price is for replacement before contemplating replacing them

Egr valve stuck, need to replace that, the car will run with bad o2 sensors

O2 sensor won't keep it from running,EGR closed won't either ,it's something else.Take off the fuel cap,have someone listen at the fuel filler when you turn the key on,should hear the fuel pump run for about 3 seconds.
If it doesn't ,we go on from here.
If it does ,check for spark