As many others already mentioned, battery is always the first thing to check with.
Ask someone to do a jumpstart, and you will know whether it is the battery problem or not. If your car has no problem to start in this way, then charge the battery up (such as to drive the car for > 30min on highway) or, if it is very old, replace it.
There are many possibilities to make your battery level low, such as leaving the car in garage for very long time, didn't drive the car very often or dirving distances were very short, door was not completely closed (warning light on), leakage in wet days (when the cables are aged), etc.
Only when you are certain that it is not the problem from battery, then go for finding other causes.