The process called decarboxylation is in reality just the act of heating the THC-crystals prior to consuming them. The reason for doing this is that several chemical processes happens in the cannabinoids when they are heated. The psychoactive properties of the cannabinoids are multiplied many times by decarboxylation. This happens "automatically" when you smoke it. It is adviced to decarb THC and CBD (and all the other "brandings" that will soon become common knowledge). That means that decarbing also activates and amplifies the medical/healing properties of the plant. It also makes it taste alot better. IMO.
If you are looking to use marijuana as a medicine, you should consume it in a decarbed state, smoking it is not effective for body-healing purposes.
The way to decarboxylate is very easy, put your grinded weed in the oven at 110 C/230 F, and leave it there for at least an hour. I usually just leave it with heat on for 3-4 hours, then I turn off the oven and leave it there overnight. You will wake up to a nice smell in you entire house. This smell can be countered by putting bowls of water and vinegar (20 parts water, 1 part vinegar 5-7%), this works for removing any kind of smell. The weed will then be brown and smell very mildly and unintrusive. Now you can just eat it as is, or mix it with whatever you want. It is easier for your body to process if you mix it with oil or fat before consuming it. I usually mix it with butter or coconut-fat, and make cakes, brownies or chocolate.