Baking, versus cooking, is more of a science and chemistry-related. You are working with different types of heat, acids, and leaveners. When working with a new recipe, it is easy to be confused with "what is what" and what can you subsitute for different ingredient.
When you have a flavoring ingredient, such as zest, chocolate, or an extract, subsituting it for another flavor ingredient will just change the flavor profile. This is perfect for changing up a basic shortbread cookie into an almond-white chocolate-apricot shortbread cookie! Turning a chocolate mousse recipe into a cinnamon-spiced chocolate mousse. Perhaps even a simple buttermilk biscuit recipe into a cheddar jalapeno biscuits!
The tricky part is when you are dealing with anything that will change the composition of the structure of the baked good. For example: baking powder, baking soda, yeasts, cocoa powder, and any other acids (vinegar, buttermilk, lemon juice, etc.).
The best advice? Don't subsitute one for another! But if you have to...
-2 parts cream of tarter plus 1 part baking soda = baking powder
-Triple the amount of baking powder to baking soda. (Example: 1 tsp baking soda = 3 tsp baking powder)
-1 "cake" of fresh yeast = 1 tablespoon dry yeast = 1 tablespoon instant dry yeast
-Stick with your acid that your working it!
Yes, you may realize that you do not have a particular ingredient to start your baking adventure, but having a stocked pantry helps, along with practice and knowledge. I love "The Food Lover's Companion" book because it is a dictionary of all foodie/cooking/baking terms. Another tip: write and date any notes you have with the baked good on the recipe itself!