This answer only applies to pregnant women whose water has not broken, carrying one full-term baby in vertex presentation (head down).
The Braxton Hicks contractions you may have been experiencing since the middle of your pregnancy are just occasional, irregular cramps that generally subside. When true labor begins, the contractions start as mild, irregular cramps that become regular and more painful over time. You may feel these cramps in your back or in your upper or lower abdomen. You usually can't feel your baby move during the cramp or contraction. The contractions push the baby's head down, slowly thinning and opening the cervix. This is called effacement and dilation. False labor is when you feel the same pains, but the contractions do not open the cervix. It is not real labor, but it is real pain! Real labor dilates the cervix.
First labors are usually a long process. The earliest pains are known as prodromal, latent, or early labor. This phase can last a day or two, or it can start and then stop. Eat frequently and drink a lot of fluids, especially water and clear juice without too much sugar in it. You may shower and walk, but don't allow yourself to become exhausted. It's also important to rest, lying on your side with the mattress supporting your belly.
Time your contractions from the beginning of one contraction to the beginning of the next contraction (not to the end). When your contractions are regular and strong, and coming every four to five minutes for one to two hours, you should call your midwife or doctor. (A contraction is considered strong if you can't talk through it.) She will ask you several questions that will determine if it's time to go to the hospital to be examined. Only a vaginal exam can indicate whether your cervix has effaced and dilated. If your cervix is open approximately 4 centimeters (and hopefully effaced), you are in active labor and will be admitted.
If you are preterm, pregnant with twins or multiples, or have other high-risk conditions, contact your doctor or midwife immediately if you think you might be going into labor.