The labor pains start when the woman is ready to give birth to the child. The uterus contractions cause this pain, pain increases as the cervix begins to dilate. This may take hours or days. With each contraction the cervix dilates more and when it is completely dilated the baby can come out. So, the pain is intense when this dilation happens. There are medications available to deal with this pain and many women choose to take these pain-medicines.
Looks like you are a very curious-teenager. If you are about to be married and if you have questions in mind, then I would suggest you to consult a gynecologist to have a discussion on your doubts. All the best.