Here are some of my favorites: avocado, onions (red onions are very yummy!), lettuce, tomatoes, bananas, almonds, walnuts, beef, pork, chicken, vegetable juice low sodium, and water with flavorings.
We had tacos last night for dinner. It was so good with the whole wheat wraps and a lot of vegetables and beef. So we got a lot of protein, fiber and other vitamins. Avoid foods with a lot of sugar--just went to the grocery store on Saturday and there was yogurt with 28 grams of sugar for one! That is way too much sugar. Avoid a lot of juices, regular soda and refined grains. The sugars program your body to eat and eat more sugar, so you gain weight and keep getting sugar highs and crashes. Look in the ingredient list - do you see anything resembling sugar? A lot of times they give it fancy names raw cane sugar, brown sugar, high fructose corn syrup, honey, etc. If it's in the top 5 ingredients I would think twice before ever buying it.
Going to the grocery once a month is not enough as you can't be buying the good options for produce and perishables. If you're buying a lot of processed foods, then that's what causes us trouble. I'm in the same boat like you as I'm trying to lose weight. I wish you the best!