Potatoes should not be kept in a refrigerator but rather in a cellar or another cool, dry place. The same holds true for tomatoes. Celery stalks can be set in a glass of water in the refrigerator to retain its crispness, as can radishes, but much of the vitamin content will be lost. Lettuce should be stored in sealed plastic in the refrigerator. Do not cut or wash it until you intend to serve it.
Fruit is actually quite simple.
If it is ripe, store it in the fridge. If it is not, store it loosely in a bowl or basket that will allow air to circulate around it, which will keep it from holding bacteria while it ripens. Any meat should be kept in the coldest part of the refrigerator. Be sure a little air can circulate around the meat, however it is packaged. This will help prevent bacteria from forming in too much moisture.