It's a bunch of particles added to "da" (the copula.), and other verbs/adjectives. datte/tte: emphasized (not to be confused with the tte used when quoting someone). datteba/tteba: even more emphasized, or showing impatience/annoyance when used after someone's name. dattebayo/ttebayo: -even- more emphasized, pretty obnoxious. You won't find real people saying this (unless they're cosplaying naruto.)

hi, im a naruto fan too and masashi kishimoto said that`s just something he invented to make naruto more distinctive, more of an outcast. so it really means nothing, although they translate it in mexico and u.s.a. as "de veras" or "beleive it" that's not a literal translation. so you are right, it means nothing it just gives rythm to his speech.

as far as i know it's just an expression. in the earlier parts (when naruto was young) it seems to be a childish expression. now that he is older, i see it as a show of toughness in ramen fighter miki, a guy there adds the word "nya" at the end of his sentence, i guess its something like that too