Hi, here is a general guideline for making popcorn:
You'll need a large pot. Think 'soup pot' or 'dutch oven' types of pots. 6-8 quarts ought to do.
Use about 2 tablespoons of oil - something neutrally flavoured, a la Canola, and with a reasonably high smoke point. Add the oil to the pot over medium heat. Throw a couple few kernels of corn in the pot and cover it. When those kernels of corn have popped, your oil is heated enough to start.
Add the rest of your popcorn - about 1/2 cup. Cover the pot and shake it to coat the popcorn with oil. Continue to cook, cover, and shake the pot occasionally (I shake it near constantly, but every 20-30 seconds is probably sufficient). Keep this up until the popping basically stops.
Cover with butter and salt to taste. Don't use margarine, and especially don't use 'light' margarine, the water content will melt your popcorn. Keep in mind that most of what people prefer about the taste of buttered popcorn is actually the flavour of the salt, the butter just helps it stick, so no need to go overboard with the grease, 3 tablespoons is probably plenty for this much popcorn. Popcorn is the one place where good salt is a bad idea. Use ordinary, free flowing table salt to get the best distribution of it.
If you throw a tablespoon of sugar in at the oil stage, you'll have to be much more mindful of the temperature, but the end result will be a kind of kettle corn that is also excellently scrumptious. Alternatively, just add a couple tablespoons of fine granulated sugar at the same time you add the butter and salt.
Enjoy your popcorn!