If you'd like to naturally carbonate your beer without using priming sugar or malt extract, you can take a certain amount of your unfermented wort, place it in a sanitized container, and place it in the fridge until bottling time. This unfermented wort used for carbonation is called gyle. At bottling time you mix the gyle into your beer to start the carbonation. The amount of gyle needed is proportional to the batch size and the starting gravity of the wort. The simple calculation is (quarts of gyle) = (12 * size of batch in gallons)/((starting gravity - 1) * 1000). You may use this simple calculation tool to get the amount of needed wort:

Batch size: gallons
Starting gravity:specific gravity