When a recipe calls for minced garlic, it is asking you to chop the garlic into small pieces. To mince garlic, first peel off the skin. Then, using a sharp knife, cut the garlic cloves in half lengthwise. Finally, start chopping in a back and forth motion until the garlic is in tiny pieces. One clove of garlic yields about 1 teaspoon of minced garlic. So, 4 cloves of garlic would be equal to approximately 1 tablespoon of minced garlic.
If you don't have fresh garlic on hand or if you're looking for a shortcut, you can also use jarred minced garlic. This is typically found in the spice aisle at your local grocery store. Just keep in mind that the flavor won't be quite as strong as using fresh cloves.