Carbon comes mainly in one variety, C-12. All carbon has 6 protons and 6 electrons. The 6 electrons make it carbon, and give it its chemical properties.
Some carbon has more or fewer neutrons.
One of its protons became a neutron. There is a fairly constant amount of radiation from space, mainly from the sun. And so, C-14 is being produced at a fairly constant rate. Plants take in carbon dioxide and water, and produce carbohydrates and oxygen. This process is called photosynthesis.