First thing you need to do is take the battery out and have it charged up. Without a good strong charge in cold weather, it will be hard to start. Batteries can also freeze, I've had it happen, and you are better off bringing the battery indoors, if the car has to sit outside in these temps.
Second, the car will not start unless the battery is fully charged.
Also, what kind of weight oil are you using? 5W-30 or at least the 10W the is recommended? Oil becomes very thick in these temps too.