Hi Janet, welcome...  Calcium carbonate is absorbed more readily when taken with food and calcium citrate can be taken without food.  Here's a quote from the National Institutes of Health on these types of calcium and how much elemental calcium is in both.

Calcium supplements contain varying amounts of elemental calcium. For example, calcium carbonate is 40% calcium by weight, whereas calcium citrate is 21% calcium.

The percentage of calcium absorbed depends on the total amount of elemental calcium consumed at one time; as the amount increases, the percentage absorption decreases. Absorption is highest in doses ≤500  mg. ((less than or equal to) So, for example, one who takes 1,000 mg/day of calcium from supplements might split the dose and take 500 mg at two separate times during the day . National Institute of Health 2010 .  See the previous link from the NIH for more information on these types of calcium and which might be best for you.

Good luck and don't forget you can get quite a lot of calcium from your diet, which is recommended, and see the list of foods that contain it at the above link.  If you get calcium from your diet, calculate your totals of that first and then supplement, if need be, with oral calcium supplements to total what your Dr recommended for daily intake.  I'm able to get most of my calcium from the foods I eat, and some feel this is the best way.  You rarely hear of problems from calcium, like constipation, over-dose etc from calcium-rich foods.

If you have another question don't hesitate posting again