i haven't paid close attention to the lyrics so i don't know what the implication of the subject is, but yes, it's either "it starts from here!" or "I start from here!"
the "n" is meaningless, at least from what i was taught. it's more to do with it sounding more fluid, it doesn't sound right to say it as "hajimaru da". "da" as in the informal for "desu" (so it would be "hajimaru
n desu" not "hajimaru desu").
now, i'm sure saying "hajimarun desu" has a slightly different connotation than "hajimarimasu" or just "hajimaru", but that's where my knowledge stops.

maybe it has more to do with the grammatical structure of the statement?