It is a rising agent.
It is important in most recipes but not in some others
Helps Rise
Baking powder in a recipe helps your item rise when baking.
What are you baking? In cakes it helps with the batter.
yes indeed
The recipe that I use calls for baking soda.
Baking soda and baking powder are very different in how they work. Think of baking powder as making something more light and fluffy. Baking soda is going to make something taste less sweet and slightly more light. But yes. If your recipe calls for baking soda, or baking powder using the correct amount of both is important. You cannot substitute one for the other.
Yes it aids in the rising. When a recipe contains baking powder and baking soda, the baking powder does most of the leavening. The baking soda is added to neutralize the acids in the recipe plus to add tenderness and some leavening.
One that works. What recipe you use depends on what you think the judges are looking for. However if the recipe is not "of your own making", you do have to ask yourself whether it's cheating or not (i.e you're borrowing someone else's cooking skill). On the other hand, recipe really isn't that important, so long as you have the required baking skills to match. (Anyone can make the recipe the wrong way).
To replace for example 3 teaspoons of baking powder with baking soda, mix 1 teaspoon of baking soda with 2 teaspoons of cream of tartar.
You can use self-rising flour in any recipe that also calls for baking powder. When you do use self-rising flour be sure to omit baking powder, salt and baking soda if in the recipe.
It's very doubtful it will make you sick. However, the baking powder will be useless in the recipe if it has reached (or surpassed) the expiration date.