1) When it comes to ghusl I’ve been doing it my whole life with going in the shower and fully soaking myself and then starting with my head and neck touching them and then turning my right side and then my left, but I didn’t know I should do my head and neck alone first without wetting the rest of my body then doing the right side and left is my prayer not accepted, also is ghusl al janabaa the same procedure?

2) My father is sunni and I am shia. He knows that I am shia and sometimes invites me to pray salat with him leading the prayer, but I don’t know if I should say my own shahada, and should I pray again later or is my prayer valid if he is leading

1) As far as the way you perform your Ghusul it’s correct and you don’t have to wash your head and neck first without wetting them first.

2) You can follow you father in salah in all the actions but you do your own recitation Shahada and Dhiker.