In frostings or other baking, substitute an equal portion of water or other liquid. I have used rice milk in some recipes as a straight substitute for milk, and had disastrous results. Water seems to work better for pudding, and some baking. Both are good for baking. Not all products have to be made from scratch.
Bread is a special challenge for many, and there are pre-baked alternatives available.
Rice bread, millet bread, tapioca bread, rye bread are all available. Ener-G Foods has many breads, including Raisin Bread. The Gluten Free Pantry also has many fine products, but watch for other ingredients like eggs in their mixes. There are also pasta alternatives, made with either rice, corn, quinoa or a blend of these. There are many excellent food allergy cookbooks available, some for single allergies such as to dairy products, others for a variety of allergies.