Most of the cast iron you buy today comes pre-seasoned for you -that's great, it makes getting started with it all the easier for you. Just run some scalding hot water on it and dry it before you use it the first time.
It'll last a life time and can be handed down from generation to generation.
If you have cast iron that needs to be seasoned here's what you do:
This should be the only time you scrub your cookware with a scouring pad and soap inside and out (this will help keep the seasoning even).
Wipe Crisco shortening or cooking oil all over the inside and outside.
You should have a thick film but NOT so thick that it looks like frosting, if using oil, it shouldn't pool up in the pan.
Put it in the oven at 250 F with a cookie sheet or a piece of foil underneath it and turn it upside down.
Bake it for 2+ hours.
Take it out.
You're good to go.
Let it cool before putting it away.
The more you use it, the more seasoned it gets.
This is way better than non-stick coating on you cookware. It will never peel up.