I don't know too much about the pregnancy besides the due date... Can you help me

Date of conception is mere guess work, as is the due date, in all honestly.
Pregnancy is not hard and fast on dates, but if you have paternity concerns then the ONLY to resolve this issue would be a DNA test after the birth.

Most likely September 24. However, due dates go by your LMP, so without knowing that it's hard to calculate. In the end, if you are concerned about paternity, you will have to have a DNA test after the baby is born.