Typically, soymilk can be substituted for milk in a 1:1 ratio. However, soymilk will not always produce the same results as milk (because milk has a different chemical makeup from soymilk).
In many baking recipes soymilk and soy creamer can be subsituted for milk with satisfactory results. In recipes such as chowders and soups, you'll have to be careful that you don't curdle the soymilk by boiling it -- simmering it is about as far as you want to go with soymilk (thickeners like flour and cornstarch can help with this). Remember to use plain soymilk versus a flavored variety in savory dishes. Also remember that the taste, texture, appearance, and flavor of your recipes won't be exactly the same, but you will be able to produce a satisfactory result in most cases.