Every time you use a noun in English, you need to decide two things: (1) whether an is article necessary and (2) which article is appropriate. If English is your native language, these decisions will be automatic. Otherwise, however, you will need to consider what type of noun you are using (countable or uncountable) and what context you are using it in. Proper nouns, as well as determiners (such as demonstrative and possessive adjectives), have their own article usage rules.
Keep in mind that idioms do not follow ordinary usage rules but include the presence or absence of an article as part of their form. For article usage in idioms, check a specialized dictionary.