If you are underage, I'd recommend having a parent or adult give you a hand with this.
Basically you treat it as you would an iron skillet. Mineral oil works quite well, use food grade if you ever plan to cook something in your cauldron (some kitchen witches I'm told do, so I'll include directions for cleaning if you do use it for cooking uses). You can use pretty much any kind of neutral oil, including paraffin, it really just depends on how you want to use it. The main focus is on sealing the pores in the iron. Once that's done and you maintain it well, it will be protected.
If your cauldron is dirty, you can use a damp cloth to clean it out, then immediately heat it in the oven and bake until dry.If it is rusty, soak it in a mixture of white vinegar and water (vinegar 2 parts, water 1 part) for 24 hours, then take out and gently scrape off the rust with a wooden spoon. Then wipe it out with a slightly soapy, damp towel and bake it until it's completely dried out. Then proceed with seasoning, as directed below.
Seasoning a Cauldron
Coat the pan in oil and let a generous amount of the oil sit on the inside surface for 24 hours.
Wipe off excess oil with a paper towel.
Line the bottom of the oven with aluminum foil to catch drips. Preheat the oven to 500 degrees Fahrenheit. Open the windows and get some fans ready. It is going to get pretty smoky in the house.
Bake the cauldron upside down in the oven for 1 hour.
Turn the oven off and allow the cauldron to cool to room temperature before you take it out.
Cleaning and Maintenance
Always heat the cauldron before using.
Use the cauldron exclusively for very hot cooking for the first few months. This will continue the seasoning process.
Wash the cauldron immediately after use. Use a tablespoon of salt and a splash of water, and scrub with a cloth or paper towel.
Set the cauldron on a stove burner to dry. Use low heat and leave it for just a minute or two.
Remove stuck on food or herbs by scraping with a metal spatula or spoon. If this doesn't work use soapy water and a scrubber. Dry on the burner as usual and then rub the surface with paper towel and some mineral oil.