If the word after a or an is describing something do I use the descriptor or the described to choose between a or an?

You use "a" or "an" based on the sound that begins the Word following it, regardless of whether it is a descriptor or the described noun. For example, you would say "a big Apple" because "big" starts with a consonant sound, but "an Apple" because "Apple" starts with a vowel sound. The important factor is the initial sound of the Word that follows "a" or "an."