What do I call to my father's first wife as I am from my father's second wife's child. Step mom is for my father's 2nd wife by children by first wife or it applies to both?
Since there is no specific term for this relationship, we have to go by common practice and practicalities.
I can speak from personal experience of what terms were used in the past, in England, as both my mother and my grandfather (on the other side) were from second marriages. I heard that my grandfather's first wife died in 1914 - and that was always the terminology used. Similarly I heard about my great grandfather's first wife who died in perhaps 1885.
Further, the children of these relationships were always refereed to using terms like my great grandfather's son by his first wife or my grandfather's son from his first marriage. There was never any attempt to use terminology based on half uncles/ aunts (which they were, of course).
As for the use of terms like aunt, my grandfather's first wife was referred to as my great aunt, but this was not because of any aunt-like perception - it was because she was actually my great aunt, as my grandfather married two sisters in turn.
The woman currently married to your father (not your biological mother) is the one who is your step-mother. If he remarried, the new woman becomes your new step-mother, and the previous woman is now your former step-mother. If you were not born yet, that previous woman would be called exactly as you described, 'my father's first (or former) wife'.