To get started, break up your food into groups. You can do this however makes sense to you. Some people like to categorize by how the food is organized in a supermarket, but you can also do it in ways that reflect how you use the food, such as by meal type, storage type (canned, freezer, etc.) or any other way that will help you stay organized.
Next, get your starting inventory together. This will be very time consuming, but it only has to be done once and then you will simply update as you put food in or take it out. Not all food needs to be on your inventory list. Things you use only occasionally will not serve much use in an inventory list, but the staples your house goes through on a regular basis should be included.
Once you have your list, determine a minimum amount to have of each item. If you know your favorite soup takes two cans of chicken stock, make sure you always keep at least two cans on hand.
When you use an item, mark it off the inventory list and if necessary, add it to the grocery list.
Keeping a food inventory list will allow you to always keep essential items on hand and will help eliminate over purchasing when you estimate a lower quantity on hand than you actually have.