I saw the sentence in the title of this post in my Spanish version of For Whom The Bell Tolls - Por quién doblan las campanas. It took me a minute to grok the meaning, since there are a few different grammar concepts packed into the sentence. Let's go over them.
1. Hace falta. If something "hace falta"'s , it literally makes a fault, which means it is necessary.
2. Decir is conjugated in the first person subjective. The change in subject between the clauses connected with the "que" in the impersonal expression1
3. Se is used for various different reasons in Spanish. In this case, it is being used as an indirect object pronoun. One would use "le" for the singular third person/formal 2nd person, but since the direct object pronoun "lo" immediately follows, the le is converted to se.
Put those together and given the context, the translates to : "I don't need to tell it to you."
- This comes from the I in the acronym English speakers are generally taught (WEIRDO) which tells of the various cases where one uses the subjunctive in Spanish - Wishes, Emotions, Impersonal Expressions, Recommendations, Doubt/Denial, Ojalá [↩]
Faltar is a verb you are going to want to get very familiar with once you get back to Spanish speaking lands. It seems like it gets used for everything.