I was studying adjectives in spanish the other day and I learned that adjective follows the noun. If that is the case why do we say "nueva canción" and not the opposite?
you can say both. There was a post recently that explained which adjectives go before the noun and which go after. I wish I could find it. There was a cool acronym to remember it by. It lists "age" as one of the adjectives that goes before the noun.
About your question:
• "Nueva canción: is a musical genre (a social movement plus folk songs).
• "Canción nueva" also works fine. Example: "Voy a cantar mi canción nueva para tí."
Adjectives can go BEFORE the noun in Spanish, too. They actually have slightly different meanings depending on whether you put them before or after.
Nuevo BEFORE the noun means "newly acquired". AFTER the noun, it means "newly made".
I believe that adjectives before the noun carry a more subjective (based on perception) meaning, and adjectives after the noun carry a more objective (descriptive) meaning.
Here's a link to a page that has a list of some adjectives and their meanings depending on where they are placed: before or after the noun: