Modern Greek word order is fundamentally SVO (subject verb object). The word order is flexible, though, because Greek nouns have nominative, accusative, genitive, and vocative case endings. A noun in the accusative can appear at the beginning of a sentence, for example, if the speaker/writer wants to emphasize the direct object in the sentence.
In " Στο 'χω πει", though, Στο is a contracted form of the indirect object pronoun Σου ('to you') and the direct object pronoun το ('it'). So, the sentence can be translated as "I've told it to you". These pronouns are placed before the verb.