Step One:
Look at your upcoming week. Take note of days with late work meetings, evening classes at the gym, social obligations, etc. These are the days you want to have meals ready that you can just reheat (leftovers) or meals with minimal cooking required (salads, wraps, etc.).
Step Two:
Decide on a menu for breakfast, lunch, dinner and snacks. get specific and don't be afraid to repeat meals throughout the week. Check out recipes and take note of all the ingredients you will need.
Step Three:
Survey your pantry and refrigerator to see which items you have on hand and which you will need to purchase.
Step Four:
Make a list! Write down everything you will need for your meals and snacks for the week and the amounts.
Step Five:
Go to the grocery store. It's helpful to go to the same grocery store every time you shop so you learn the layout of the store and can easily gather the ingredients you need.