"God" is a proper noun, the English name for the one Christian diety. In Islam, its name is "Allah" (which means "God" anyway). For example, "I pray to God", or "Yuki is God" Alternatively you could be a blasphemer and call it "Ya'Weh", which is supposedly God's real name.Otherwise, it's not capitilized.It's pretty simple.
By the way, it's offensive to the Jewish to say Yahweh, so you might want to consider not using it.
Everything is offensive to someone, so there's no point in trying to be politically correct 100% of the time.
U MAD?
Asking if I'm mad is like asking yourself if you breathe.