Bread from a French Boulangerie is almost always of a good quality. Look for a busy store, this is a sign of quality. Be up in the morning and be prepared to queue. As a general rule, avoid French supermarket bread - it's not much cheaper, the real stuff is of good value, and tastes nicer too.
French bread is not loaded with preservatives like English bread - buy it on the day it will be used, and keep it wrapped in a dry tea-towel - or it'll go hard and crusty and stale, particularly in a hot environment.
Milk and Butter
Fresh milk in France is quite hard to come by. Most supermarket milk is UHT, but at larger stores, a few brands of bottled milk are available. A good choice is Candia, available in normal, semi-skimmed (blue label, demi-écremé) and skimmed (écremé). As far as butter goes, buy a nice-looking packet from the middle price-bracket.
Tap water is not always good to drink in France. The French often buy 'eau de source' from the supermarket in 1.5L bottles. Note that this is not the same as 'eau minerale naturelle' - the budget-looking water can be as little as 2F per litre, around 20p. This is five times cheaper than the posh water that the Brits all buy - like Perrier, Badoit, Evian etc. The other water is pretty good, as a guide, French families will use this water even when entertaining guests.
The tap water is fine for washing up, washing and all the rest, it's just drinking it that is occasionally unadvisable. It can usually be cooked with as well.
You can go and buy tinned items for cupboard stocking in the supermarkets, they are usually quite good if you steer clear of own-brands. Any dried, non-perishable food is best bought at the supermarket - it saves time, money and effort. Tea is not drunk in the same quantity or way in France as it is in the UK, and is often designed to be drunk with lemon, not milk. It's probably best to take your own tea. As far as coffee goes, buy a nice-looking brand such as Carte Noire, now also available in the UK, and avoid the really cheap stuff. Also, don't expect to find the same brands in France as you do in the UK - the French have their own brands, and while they may taste a bit different, they're usually quite good. A good example is the Lesieur range of mayonnaise and dressings. Think of it like you would cars - try finding a Volvo dealer outside of the largest towns, but there are heaps of Peugeot, Citroen and Renault dealers.
Fruit and Veg
Buy as little fruit and veg as possible from supermarkets - go to the market, and hunt for both quality and value. Feel the produce, beware of vendors picking for you, as a tourist. Buy melons that are riper than British supermarket ones - they taste great, but if they've already split when you first see them, leave them alone. Look for local produce, and try not to buy things from abroad. Buy tomatoes on the vine - they're more expensive, but taste fantastic.