Mythology refers to stories and ideas that say something about a culure and its values. Usually myths are stories about the supernatural from the past that are not true.
The Greeks, Romans, Celts and Nordics
It is generally accepted today that the stories the Ancient Greeks and Romans believed about their gods, goddesses and heroes are myths. Few people believe Zeus or any of the others today, though there have been some revivals of Helenistic religions.  The stories associated with the Nordic religion, Odin, Thor and the others are widely considered mythology as well. Some people believe in the Nordic religion, it is called Asatru. The stories that go with the Celtic religion are also widely considered mythical though they can form part of New Age-ism.
Hindus have a wide range of stories about the gods, goddesses, heroes and heroines that they believe played a part in India’s magical past. Outsiders are unlikely to see any qualitative difference between these stories and other mythologies. Indeed many Hindu gods are related to Greco-Roman gods. People who were indoctrinated into believing Hinduism during their impressionable childhood naturally tend to believe that Hindu religious stories are true while the stories of other religions are mythological.
There was no single Mesopotamian mythology since many different cultures lived in the area at different times. Some mythologies from Mesopotamia were written earlier than the Old Testament but the Old Testament has many similar themes. Therefore it is likely that the Old Testament stories were developed from their Mesopotamian counterparts. Mesopotamian mythology was similar to the mythology of other Semitic peoples in the area and included creation mythology, flood mythology, and a hero who successfully faced an obstacle.
Mesopotamian mythology was polytheisic which can explain the use of the plural “Elohim” in the early parts of the Torah. The Sumarian god Enlil caused a flood corresponding to the Biblical global flood when he decided mankind had failed. 
Jews, Christians etc
The Bible contains any number of Fairy stories that resemble other religious traditions about
stuff that should be unbelievable wonderful supernatural events. This applies to the Old Testament and the New Testament. There is no reason to treat these stories differently from the myths of other religions. People who have been taught Christianity, Judaism or Islam during their impressionable childhood frequently have difficulty seeing that their mythology is as reasonable or unreasonable as other mythologies.