I sorta just go with what I have in the house/craving. Experience shows you can surprise yourself what you can make with a bunch of leftovers!

Like others say, making a weekly menu is a good idea that way you limit your grocery shopping. Also, try to reuse ingredients or meals. Like if you have a roast chicken one night, then chicken sandwich or soup the next. Also making something big and then storing portion in the fridge and freezer. Like make a huge lasagna for supper one night then take the leftovers for lunch the next day or freeze them for future meals. I also base what I buy on sales. So if ground beef is on sale, I will buy a big bunch. Make hamburgers one day and maybe pasta the next, or whatever.

As for online tools, I do not really know any, sorry.