Dal is an essential ingredient in Indian cooking. At our house, these three dals, split yellow dal, green split moong dal, and red dal are the most common. I've found all three at stores like Whole Foods, but they're cheaper at Indian grocery stores.
When I got married, I thought that flour came in two varieties: white and wheat, and while you will need both white and wheat flour to make some Indian dishes, they are by no means the only kinds of flour you'll need. One of the most essential kinds of flour in Indian cooking is called atta: it's used for making all of the delightful Indian flatbreads including roti and parathas. The other kind of flour that I use a lot is called besan (gram flour). Besan is used as a thickening agent in all kinds of Indian stuffing, and it is also used to make certain kinds of parathas, including my very favorite, theplas. This is by no means an exhaustive list of the various kinds of flours used in Indian cooking (or cooking in general, I know), but these plus white and wheat are my go-to's. Atta and besan are available at Indian grocery stores everywhere and online.