Summarize the following:
If you plan to take the supplement with food, calcium carbonate requires food to properly digest and absorb. Calcium citrate can be taken without food. Some supplements contain both, so as with any supplement, always read the label.  As with all supplements, calcium supplements are not regulated by the Food & Drug Administration (FDA) for content or potency. Look for a third party verification, such as the United States Pharmacopeia (USP). Always talk to your doctor before adding any supplements to your diet. Your doctor can help you decide what dosage you need based on your diet. The recommended amount of calcium per day for an adult is 1,000 milligrams, but everyone is different. Calculate how much calcium you receive from food and use supplements to add to that. Women over the age of 50 need 1,200 milligrams of calcium per day. Vitamin D goes hand-in-hand with calcium absorption. Without vitamin D, your body will only absorb 15-20% of the calcium you consume. Limited sun exposure, approximately 15 minutes, will provide your body with ample vitamin D. The amount of time you expose yourself to the sun is important as burning your skin doesn't allow you to get the proper benefit from the sun. Wear sunscreen to help you avoid burning.

Summary:
Research different types of supplement. Take the correct dosage for your needs. Increase your vitamin D intake.