When i turn the key it wont start. Instead it makes a clicking sound.

click sound where?

I have same problem change starter and I just get one click not multiple as of if battery dies charge my battery today put it back in my Buick terraza which also has on dash where I can flip through in and it will let me know the voltage it was at 12.7 Crank that a couple times when not start still got the one click after a couple times battery voltage went all the way down to completely to 12 what could be the problem and how could I fix it