What is the best way to maximize shopping budget?

I was a bachelor and seldom shopped for food. Now, I got my GF and her 14-year old son living with me and she wants me to start shopping and paying for food. It blows my mind how quickly the tab adds up. Since I am new to all of this .. I really need some help from you seasoned shoppers ... any tips or ideas ... how can i cut my bill down .. please help?????