Hello 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
This discharges any capacitors / memory within the ECU and around the car so ensures all memory settings are wiped clear - back to the factory default.
This will reset all the systems on the car and you may need the radio security code to get it going again, and ensure that the keys are in your hand when you re-connect the battery as the car may lock and turn on the alarm