Get a diagnosis. Make sure your dizziness is not due to some problem with your heart, dehydration, or the side effect of a medication. Once those are ruled out by a physician, pay attention to when it happens. Sometimes if a pregnant stands up too rapidly or stands in one place too long, she can get dizzy. Make sure you do not lay on your back because the baby/uterus can compress your aorta and lead to fainting.
Determine cause. Two serious causes for dizziness need to be evaluated: hypertension and diabetes. If these are not present, then it is probably due to poor blood flow to the brain. Blood pools in the legs and abdomen. Keep your legs up and take your time when standing to allow blood to get to your brain. Some have migraine attacks with spasm of arterial flow to the brain. Get a thorough medical evaluation.