In our household we have a whiteboard on the fridge. As we run low on items we add them to the ongoing list. It helps a ton because whenever I'm actively making a grocery list or meal planning, my husband never has any ideas what he wants to eat, but sometimes he gets a hankering for things when I'm not around and he can just add it to the list. Last night he told me he wants quesadillas for dinner tonight so I'm going with Mexican food all week so we can reuse ingredients. Tacos tomorrow, southwest egg scramble, baked potatoes with leftover taco meat(veggie faux meat), or taco salad fixings on a bed of lettuce, whatever it takes to use up all the perishable ingredients.
First, it's never as simple as just buying groceries, it also involves active family meal planning and a day to day understanding of what your food needs will be depending on the household schedule. How many breakfasts, lunches, snacks and dinners do you need to shop for? How often do you eat out? Does anyone eat at work/school? Does anyone have a special occasion/field trip/work event coming up where meals will be provided? Are you going on vacations, is there a holiday coming up?
Who is going to cook all the lovely meals you plan and buy for? How long does all of that take?
In our home I do all of it, the planning, the shopping, the prep, and the clean-up. I love it, it's like a little game to me.
Sample weekly meal plan and grocery shop with subsequent meal prep would be something like: 1 semi large shop at either Fry's or Target, with 2-3 mini shops for the things I forgot and/or fresh ingredients. I generally need to provide easy to prepare lunches for the husband that he can do himself, breakfast for myself, snacks and lunches that I can take to work for myself, and dinner for the both of us. Dinner gets complicated because I'm vegetarian and DH is not. So sometimes I do meatless meals, and other times I do a separate protein for each of us with same side dishes. After the larger grocery trip I do quick preps to keep things fresh and organized, with meals planned written on the white board, and vegetables and fruits washed and cut up. I can happily eat the same thing every day for breakfast and lunch (and often do) but DH hates that, so he usually has dinner leftovers set aside for him or else junky food type things like microwave mac + chz or ramen noodles.
I keep a binder filled with some of my favorite dishes, but it's also good to have a rotation of vegetables that everyone likes in mind. My dinners usually consist of a protein, some starch, and a vegetable. Actual examples are: Sesame fried tofu with broccoli and rice, or Eggs prepared any way with salsa, avocado and half a baked sweet potato, or faux steak strips for me, real steak for DH with skillet red potatoes and green beans.
I love broccoli, DH loves brussel sprouts, we rotate through side vegetables so no one gets too bored. I love roasting vegetables, its easy, if you foil line all your baking sheets clean up is quick, and it's a type of cooking where I don't have to stand over the stove and watch it, just set the timer for 20 mins, do a quick check, flip things and wait another 10 mins.
I second costco as a great place for the big things, we buy most of our household items there, such as toilet paper, papertowels, dishwasher tablets, etc. I also plan to pick up some perishables that we go through quickly on our every other month costco trip, like yogurt, and some vegetables/fruits.
Loads of people have success with online grocery ordering and delivery, as well as once per month big shops and then 1/week small shop to pick up fresh perishables (milk/yogurt/fruits/veggies). One such person I'm thinking of is,
http://www.penniesintopearls.com/. She just did a whole segment on amazon grocery delivery and another grocery delivery where it's all online and they even deliver fresh items. It requires a lot more planning though. I usually only plan a week in advance for our meals, but if you want to do once per month shopping you have to plan the entire month in advance or you'll find yourself back at the store picking up one off things for things you hadn't planned for.
Best of luck to you! I think it's wonderful that you're jumping in to help, you might even find you like it and continue doing it for the household long term, especially if you find a great strategy with online ordering and it helps your family save money.