I can give you the Sunday School version. For Catholics, God is a supreme being made up three "people" or parts? God the father (the God that created everything), God the son (Jesus Christ who died for our sins), and God the Holy Spirit (basically the embodiment of the love and unity between the two of them).
Guessing at God's history before the creation of the world is the kind of shit the human brain isn't really equipped to handle, but I guess there are probably some weird ass theologians with theories.