After you unscrew the valve stem, tap it gently to break the seal. Then make sure your pump head is fully seated and straight on the valve. Depending on the pump, that might take a little finagling. Personally, I think it's easier to have the valve stem at 12:00 rather than at 6:00 as pictured. Then if you need to, you can hold it with one hand while pumping the first 1-2 strokes with the other.

Both my pumps lock by pulling the lever inline with the valve, perpendicular to the hose. If you're not sure, you can check by looking at the pump head while pulling the valve. You'll see the rubber gasket compress when it's in the locked position.

If you're hearing all that hissing, chances are air isn't going into your tires at all. Worst case, deflate your tires until they're moderately soft but not flat. That'll make it easier to break the seal and get the pump head seated.