Fruit and veggies - mostly veggies. Sensible basic stuff like brown bread and weetabix (not in the same bowl). Cheap protein like chicken, bacon and cheese - stuff you can 'spread out' in rice, pasta, baked potatoes etc. Dont buy expensive brands - go for 'basic' or own brand in things like beanz, tinned tomatoes, rice, pasta, cereal etc.
Work out when your supermarket marks perishable stuff down (and where that bargain shelf it) ie. after 4pm on a Saturday - only buy stuff you will actually eat and that you would buy anyway - dont buy 'treats' even from this shelf.