We have three articles in the English language: a, an, and the. The words a and an are the indefinite articles. The word the is the definite article.
Indefinite articles—a and an—are used only with singular nouns. These words denote a single unspecified thing named by the noun. Thus, a pencil refers to any pencil, one not designated or revealed by the context. If the reader knows which pencil you're talking about, then you would use the pencil.
If you want to refer to an entire class, you simply use the plural noun and omit any article. Thus: “The students used pencils to take the test.”
Use the indefinite article a before nouns beginning with a consonant sound, an before nouns beginning with a vowel sound. Thus:
a house an hour a university an umbrella a UFO an FDA hearing