Had unprotected sex on 8th July 2012.the girl is 14 weeks pregnant.am I the father.last menstrual period was 15th June 2012.

Impossible to tell with such scant information.
If there is a concern about paternity then a DNA test will demonstrate who the father actually is.

From the dates you have given it is a strong possibility. She would most likely have been ovulation at that time.

You will have to wait and do a DNA after the baby is born!