Cooking by itself does not change the caloric value of a food. For example, if you put a potato in the oven and bake it, it will have the same number of calories cooked as it did before it went into the oven.

However, two things may intervene when we cook food:
1. the food may shrink. In fact it will probably shrink. 100 grams of spinach, for example, takes up much more space raw than it does cooked, but the same thing happens in a less dramatic way with other foods; this is why weight is a more accurate measure than volume when counting calories.
2. we frequently add a fat, sweetener, or other flavoring agent when cooking foods, so we need to add the caloric value of these to the food we are flavoring.

Hope this helps.

There are many calorie counters on the web, some of which give caloric values for foods raw, cooked, in grams, cups, and ounces; they make it easier to figure out the caloric value of your own recipes. One I use is at http://www.acaloriecounter.com/