I provide all the care and love to my fiancés daughter. He is in jail and the mother is unable to provide for her and rarely sees or asks about her... how do I go about getting custody or something?

File with the court for custody. Until then you have no rights. Be aware you will need a lawyer.

Will the mother be willing to sign over guardianship ?

Try first for temporary guardianship. That will be easier to obtain. Right now you have no rights towards the child and can't even get her medical care.

It would help to get an attorney, but at least go down to Family Court and get info on how to file.