Hi deedee
One of the problems of dating in general is that geologists rely on the Principle of Uniformitarianism:
"The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."
http://en.wikipedia.org/wiki/Geology
Carbon 14 has a "half life" of about 5,700 years... which means that half the radiocarbon in Carbon 14 decays about every 5,700 years.
So if something they find has 1/4 of the "expected" level of Carbon 14, then it is deduced to have gone through two "half lives", and is therefore about 11,000 - 12,000 years old.
Obviously this method has several assumptions. For instance, it assumes we know how much Carbon 14 it should have had in the first place.
Many creationists (myself included) believe that there was more carbon and oxygen present in the pre-Flood atmosphere. This is
not taken into account by geologists - they just
assume that "the present is the key to the past" and that the amount of Carbon 14 in the atmosphere today is the same as 10,000 or 50,000 or 500,000 years ago.
That is the problem with carbon dating pre-Flood items. If the carbon level was different in the pre-Flood environment, then we cannot use today's carbon level to measure age.
Incidentally, there are a few reasons why creationists believe there was a big difference in the pre-Flood environment. First, there was probably much much more vegetation and trees. Second, things were probably much bigger. (Some creatures literally don't stop growing until they die, and some animals that have been found are huge!)