Plan you meals for the week and then just buy what you need for them
There is a thread on here with SW staples but if you just go out and buy what everyone else says it will be really expensive. Much better to plan the week and just buy what you need. It also depends what plan you are doing, what you like eating and what your budget is like.
If you have one nearby get your fruit and veg from Aldi - we have started doing this I go Sunday/Thursday, on average we are saving around 30% a week on the more "mainstream" supermarkets.
Get in spices and vinegar and worcester sauce for your salads and oxo cubes - I am having one of these instead of a cuppa soup now.
Eggs, quark cheese, ff yog,veg, mushrooms, turkey mince, fruit, mullers, lean lamb mince ( to make kebabs) and a bottle of wine lol. ohh im getting hungry now.