I met my soon to be wife when she was pregnant she gave birth to a baby girl who is my life and now we want to add my last name legally.. how do we go about this she has no one but me as her father ,that's all she's ever going to know and we don't know who or were the bio- father is " kinda better that way" anyway we want to change her name before she starts school she's 2 right now we don't want her to question our relation in the future so the same last name sounds right I mean she is my daughther after all. How do I go about this and were? We live in cali.. please help..

Her birth certificate is blank under father
She has my fiancées last name

First sorry but you are totally wrong in "that is all she will ever know" first that is not the truth, and if and when she finds out the truth she will lose a lot of trust and faith in you both as parents, I have seen this issue cause a adult child to stop all contact with the parents for years.

Next you can not just "add your name" you will have to adopt the child, this means that there will have to be an effort to find the bio father and get him to sign over his rights.
** and unless she had sex with a lot of men during that time, yes she knows who it is, ( or at least can narrow it to two or three who could be tested )
If the father can not be located using search approved by the courts, then it would be published in the newspaper in attempts to locate the bio father.

So it starts with an attorney to start adoption procedures and searches for the father