If the car's not cranking then first check it can turned over by hand so double checking for anymechanical issues
If OK then check that the main battery cablesare on tight and the terminals are clean and bright. Do the same for the engine/ gearbox earth strap.
Then follow the main positive lead back to the engine and down to the startermotor, and check the connection at this end as well.
There should alsobe a small wire to the motor which should have 12V when you're turning theignition switch to crank. If the 12V isn't there, check back along this wirefor damage to the insulation or corroded connections
This 12V feedcomes directly from the ignition switch so this may be faulty and needreplacing
Check theresistance of the main positive lead that runs down to the motor as anythingmore than a few ohms and it should be replaced
Don't forget totry a jump start from another car / battery as its possible the battery ispoor, if you have a voltmeter a healthy battery should read 12.7+V
also trysubstituting the battery for another as I've known a very flat battery to dragdown the system voltage too far even with jump cables
one other thing to consider is that the immobiliser is faulty and preventingthe starter from being enabled - however if the dash / ignition lights are onthen this usually means that its off
If this is all OK then its possible the car needs a new starter motor