Cast iron comes either pre-seasoned, or not. If it doesn't come seasoned, then you have to season it yourself before using it for cooking. The seasoning is the natural non-stick surface that builds up. Without it, your food will stick like mad and the pan will rust, defeating the purpose of your healthy cookware.
You also have to keep this seasoning maintained, or you'll run into the same sticking and rust issues over time. Don't worry, it's super easy to do!
After cooking, DO NOT use a scouring pad or steel wool or anything on your cast iron. Just use a soft cloth or sponge, and minimal scraping. Some people don't even scrub their pans, but if you like to start fresh with no left-over flavors just give it a light scrub.
Once the pan is clean, dry it right away and put it on the stove on minimum heat for 2 minutes or so. This will evaporate any water left from washing.
When it cools a little, put a tiny amount of oil on the pan, a quarter teaspoon should do it, and rub it over the full surface with your hand. The bit of heat makes it absorb into the iron.
The oil makes great moisturizer after doing dishes, so don't be scared of it. An oil sprayer (not aerosol, please...) works wonderfully for this jo