What are my rights as the father if my daughter’s mother died?

First, if you have established yourself as the legal father then you have the right to custody immediately unless the grandparents have a guardianship order. So, you should have custody of your child now in the absence of a guardianship court order. If the grandparents file for guardianship then you can oppose it contending that there is no need for a guardianship and that you are fully capable of caring for your child.