A and The are two different forms of the same word: the indefinite article a that is used before noun phrases. Use a when the noun or adjective that comes next begins with a consonant sound. Use an when the noun or adjective that comes next begins with a vowel sound. Remember that what matters is the pronunciation, not the spelling.
Here are some examples of the article a followed by words that begin with consonant sounds. In two cases the word after a starts with vowel letter, but the pronunciation still begins with a consonant sound.
a football a driver’s license
“The” is typically used in accompaniment with any noun with a specific meaning, or a noun referring to a single thing. The important distinction is between countable and non-countable nouns: if the noun is something that can't be counted or something singular, then use “the”, if it can be counted, then us “a” or “an”