Best time to have sex with lowest chance of pregnancy

RIGHT when her period ends, but even then as a teen its risky if your periods are irregular

This is what a friend told me. She has been a nurse for several years.
put your finger in the vagina and feel what position the cervix (opening to the uterus) is in.
If it is pointing down with the vaginal canal you have a better chance of getting pregnant.
If it is slightly curved up you have a less chance.

In general about 12-days prior to a womans period starting and 7-days afterwards are the least possible times to get a woman pregnant, however, this isn't a perfect thing as womens cycles sometimes change do to varying factors (diet, stress, etc.). You could have intercourse during her period which would be pretty safe I'm sure.