If the car's not cranking then first check it can turned over by hand so double checking for any mechanical issues
If OK then check that the main battery cables are 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 starter motor, and check the connection at this end as well.
There should also be a small wire to the motor which should have 12V when you're turning the ignition switch to crank. If the 12V isn't there, check back along this wire for damage to the insulation or corroded connections
This 12V feed comes directly from the ignition switch so this may be faulty and need replacing
Check the resistance of the main positive lead that runs down to the motor as anything more than a few ohms and it should be replaced
Don't forget to try a jump start from another car / battery as its possible the battery is poor, if you have a voltmeter a healthy battery should read 12.7+V
also try substituting the battery for another as I've known a very flat battery to drag down the system voltage too far even with jump cables
one other thing to consider is that the immobiliser is faulty and preventing the starter from being enabled - however if the dash / ignition lights are on then this usually means that its off
If this is all OK then its possible the car needs a new starter motor
Also check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.
if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement