'god' is a word for unnamed and unknown gods, 'God' is used for a god who/which is usually the only god, or for the number one god, depending on the belief system. People who have no religion and no god generally don't bother with the 'G' god! :)

Here are some examples of use. 1) "“I am the Lord your God, who brought you out of Egypt, out of the land of slavery. You shall have no other gods before me."--the Bible, Old Testament (traditional holy book of Jews and Christians; modern English translation). 2) "There is no god but God, and Mohammed is his prophet."--Customary English translation of Muslim credo 3) "From too much love of living, From hope and fear set free, We thank with brief thanksgiving Whatever gods may be That no life lives for ever; That dead men rise up never; That even the weariest river Winds somewhere safe to sea." --Swinburne, "The Garden of Proserpine"

god means you are referring to a particular god in mythology. God on the other hand is referring to the single Creator in a number of monotheistic religions. (Both references are capitalized at the beginning of the sentence like any other word)

You asked in a follow-up: "BTW between 'oh my god' and 'oh my God'", which one would you use?" I would only capitalize the G and spell it "oh, my God"--probably with a comma after "oh"--if the words were spoken seriously in a state of strong emotion, a state of horror and despair: "In an online video montage called 'September 11, 2001 — As It Happened,' the voices say, 'Oh, my God.'" When used casually I would probably write it as "ohmigod." That is, I would probably write "OMG stands for 'ohmigod.'" If I did decide to write "OMG stands for 'oh my god'" I would not capitalize the g.

god is a human made but God is divine