Hi Andrea,

My understanding is that s'intéresser à is used to express the idea that someone is interested in something, where someone is the subject of the sentence, while intéresser quelqu'un is used to express the idea that something interests someone, where something is the subject of the sentence.
Examples of s'intéresser à:
• Paul s'intéresse à la littérature française = Paul is interested in French literature.
• Je m'intéresse aux vieux livres. = I am interested in old books.
Examples of intéresser quelqu'un:
• La littérature française intéresse Paul = French literature interests Paul.

• Les vieux livres m'intéressent = Old books interest me.

I hope this helps :)