When you get it you will need to clean with warm soap water and a scrubby pad to get off any rust. This will be the only time you will use soap to clean the pan. Rinse well and then dry thoroughly.
The best way to season a cast iron skillet is to rub it well with Cannola oil or another light oil. Once you have done that place the oiled skillet in the oven at 300*F for 3-4 hours. This will season the skillet and it will be ready to use.
This is a your Tube video by Bobby Flay, the Iron Chef/Food Network, explaining what I just typed : http://www.youtube.com/watch?v=74pHH_6fuYk&feature=fvwrel
This is a link to help you with cleaning the skillet after using: http://www.ehow.com/how_4910864_clean-cast-iron-griddle.html