My wife and I are "list people." We write to-do lists, items-to-pack lists (for vacations), and, most importantly, grocery lists.
If you want to be healthy or lose weight, then a grocery list will be your best friend. At home, away from the aisles of tempting food products, you'll be able to write a list of the things you need... and avoid the bad foods you want.
Once at the grocery store, your grocery list will keep you focused. If you can, organize your list so it puts you on a logical path through the store. This extra step will keep you from wandering by food that may cause you to lose resolve—food you intended to avoid altogether.
After you've gathered all the items on your list, proceed to check-out. Don't stop in the ice cream aisle and try to remember any "extra" items you may have forgotten. The longer you stay in the grocery store, the more likely it will be that you'll buy unhealthy foods that you'll regret buying later.
That's why you need grocery lists: to keep you focused; to prevent you from buying junk food; and to ensure you buy healthy foods. So do your grocery planning at home. You'll be amazed by how quickly a simple thing like a grocery list will help you start to improve your health.