The supplements should be taken at meals and should be combined with an exercise program. Here are some answers to your latest queries. Don't rely exclusively on supplements. Get as much calcium as you can from foods.
Foods contain other important nutrients, some of which promote calcium absorption.
Some kinds of tofu and soy milk contain added calcium, though this isn' t as well absorbed as the calcium in cow's milk. It makes little practical difference, as long as you take enough. No pill is pure calcium. Plain calcium carbonate, as found in some antacids, is most common and least expensive, and contains the highest concentration of calcium per pill. Thus, to get a given amount of calcium, you can take smaller pills, or fewer of them.