Depends what you like to eat. Eg having dried pasta and a jar of your chosen red sauce you can throw in leftover chicken and/or veg and have an easy meal. If you like curries and stir fries it makes sense to keep rice. If you want to bake, then flour and sugar. For breakfasts things like cereal, jams, peanut butter, eggs, and keep some bread in the freezer for toast. Start small and build up what you need, and when you use something up put it on your shopping list to replenish. Buy quantities that you will use in a reasonable time. Buying in bulk can be cheaper but if it’s only one person you don’t want items going stale waiting to be used. Not everything is equal in being shelf stable, for example cooking oils can go rancid, so keep an eye on the expiry dates and don’t overdo the bulk buys.