Stored Procedures..

Hi all,i need a introduction..or how to use that?And First of all what is that?How can i create that?I have a problem here that i need to multiply two columns and assign the result to the third...For example : I need to multiply the number of goods(shirts and pants) depending on the customer,make his bill according to his shopping tariff...On thw whole to calculate this transaction i need to implement a certain code....plz help its urgent now..