The "screw at the bottom" is actually a nut and it should be tightened against the rim once the valve is straightened out. Your valve is protruding from the rim at an angle.
If the guts of the valve will not tighten further by turning the tool clockwise and the tire will still not hold air, I would say the valve is shot. Take the bike to your local shop and have them show you how to put a new tube on.
Eric