To my novice bakers and cooks: I feel responsible to emphasize the importance of following a recipe--especially if it's your first time making something! One of the most important parts of a recipe is the ingredients. Good food comes from good ingredients. Most recipes for baked goods call for unsalted butter. Though you can use salted butter as a substitute, unsalted butter has a better flavor for baking.
Most recipes for baked goods assume that you will be using whole milk. You can almost always substitute it for other types of milk. I've got skim milk in my fridge, so I use skim milk. I've also used rice and soy milk with no problems. It won't taste exactly the same, but it will cut down on the fat and calories in whole milk.
Almost every baked good recipe calls for vanilla extract. I don't use imitation vanilla extract. I use "pure" vanilla extract, but you can probably get away with the imitation vanilla extract.
Some ingredients are NON-negotiable, unless you have a chart with substitutions. Altering any of the following ingredients may cause your baked good not to rise: baking powder, baking soda, and eggs. Also, an ingredient that may be unfamiliar to you may be the key ingredient in the dish (e.g., cilantro in tortilla soup or fresh sage or Marsala wine in chicken marsala).
Lastly, different oils burn at different temperatures. For example, a recipe may call for vegetable oil, but you've only got olive oil. If you are just sauteing something for a short time, it is probably OK to substitute. If something needs to be sauteed for a long time, be sure to use the type of oil that the recipe suggests. Also, the extra virgin olive oil that the pesto calls for is NON-negotiable.
If anyone else has any tips on ingredients, please leave a comment so we can all enjoy!