Welcome to the Wefugees Community and thank you for reaching out to us.
You did right to take care already of the recognition of the fatherhood ("Vaterschaftsanerkennung") and the declaration of the custody ("Sorgerechtserklärung"). If you want to move to your partner and child, you generally have to submit a so-called "Umverteilungsantrag" at the immigration office. If I am not mistaken, the Bezirksregierung (district government) Arnsberg is responsible for this procedure in North Rine-Westfalia (NRW) though. Please see this website for more information - they are also available in English. On the right side you find the formular (Antragsformular Umverteilung) for the respective application.
As father of a German child, you may apply for the residence permit according to section 28 of the German Residence Act. I was not able to find good information about the respective requirements at the Paderborn immigration office's website, but for a general overview you could check this list compiled by the authorities in Berlin.
In general, I would recommend you to contact a migration counselling office for support. The immigration office will eventually ask for a passport and probably also wants you to apply for a visa (family reunification) at the German embassy in Accra. Therefore, some individual counselling will be helpful.
I hope these information are useful and please don't hesitate to get back to us with any further questions.
All the best,