What initially confused me was the placement of "important" which is different from where most adjectives are placed (attributive position - before the noun).
I wondered if that was part of what was troubling you. It's true that, with most nouns, attributive adjectives precede rather than follow the noun. However, with pronouns like "something," "someone," "anything," "anyone," "everything," "nothing," etc., the attributive adjective always comes after the pronoun. The following is totally ungrammatical: *"I have important something to tell you."
Would I be correct in saying in the sentence "I have something to tell you", "to tell you" is an infinitive (adjective) ?
"Tell" is an infinitive. "To tell you" is an infinitival clause. It is functioning as a non-finite relative clause. It is not an adjective; however, like all relative clauses (e.g., the underlined one in "I have something important that I need to tell you"), it functions as an adjectival modifier. Not all adjectival modifiers are adjectives. Nouns, prepositional phrases, and clauses can be adjectival modifiers, too.