I have raised my daughter (step) since birth her biological father has never seen her or contributed to her upbringing. Me and my partner are looking to be married in the next year. What are our best options in making this happen?

Give it about a years time of being married and then apply for an adoption. The bio dad will have to be involved in some way during the process. Do not attempt this without a lawyer to guide you through it. The bio father can be dealt with on many levels and that is why you need a lawyer to guide you because this is something you want to remain permanent.

Is your partner the child's mother?

One would think so, but something about the way you ask the question makes me ask.

Consult a local Family Law attorney. They can advise you of the requirements you will need to fulfill for the adoption. These vary by area. You will need to get the bio father's agreement or at least try. You will probably need to be married for a period. And it would be best to have an attorney prepare the paperwork.