Do you have a simple volt meter at home? If so…
In the morning before doing anything, test the voltage across the battery terminals themselves, should be above 12.0 volts. Then, test voltage across the cable ends at the battery, should be the same. Then, have someone try to start the car while you are still measuring voltage across the cable ends. If the car turns over slowly and the voltage drops below about 10.6 or so, you probably have a weak battery or poor connections at the battery. Repeat the test with the meter on the battery terminals themselves to verify no voltage drop at the cable ends.
If the car turns over slowly and the voltage stays high, you probably have a bad starter.
There’s more detailed testing that can be done, but this should get you in the right direction.