how can I fix my Discovery?

By what you have said, The problem sounds like starter motor related. First of all though check the earth connections. There should be one to the body and one to the engine. Easy way to check this connect jump lead from battery negative and a good clean connection to body. Then run from battery negative to engine block. If the engine starts then it's an earthing fault.
Best way to confirm if starter is ok is to remove and test. One jump lead from negative on battery to starter body. Then one from positive on battery to main power connection on starter and one from there to small spade connector.
Don't let the two jump leads touch one another.
If starter spins round ok then must be ignition barrel. In this instance you can run a 30 amp wire from ignition barrel connection straight to spade connector on starter.

chris
December 2005