I am not too clear on what can be used and what is not in a cake? Also the icing? Any help would be great!

Are you're talking about making a Kosher cake for the Bar Mitzvah?

Unless YOU have a Kosher kitchen, then you can't produce a truly Kosher cake. The kitchen, ingredients, processes, etc. have to be certified/Koshered by a rabbi. You can purchase Kosher ingredients (packaging is marked "pareve", a U inside a circle, etc.), but unless made in a Kosher kitchen, the item isn't actually Kosher.

You probably neeed to verify with the customer that they need the cake to be Kosher. If they do, they'll need to find another baker. If they don't/don't care, then regular ingedients are fine. If they just want Kosher ingredients then you'll need to know what symbols to look for and then inspect all of the packages of your ingredients very carefully.

Good luck.
Rae