Where does water come from?

With all this rain we’ve been having (it rains every single night! Sometimes it flat down pours for a couple of hours) I’m wondering where it all comes from. This takes me back to when the earth was just forming. Our atmosphere developed, then it rained for a million bazillion years and filled up the oceans.
Where did that first water come from? How was it created? Can water be destroyed, and can water be created? Could we ever run out of water?
Does the total amount of water running through the evaporation cycle remain the same?