I'd wager that you are:
the answerer (me, in this case)
I've been here before I became me
The answerer visited and viewed this question before answering and becoming the answerer.
I'm wise and bold, you're about to see
While there can be numerous respondents, only one is wise (or bold/quick enough) to be the true answerer with the true answer.
Carly Simon, it's not my sin
Carly Simon sang "You're So Vain- you probably think this song is about you..." Well, this puzzle is about me, since I am answering it, becoming the answerer.
I'm the one who'll have the green
Since I am the answerer, I will be the one to receive the green tick.
All of you, who are reading this
Will know my name if you persist
If you continue to read down the page to this answer here, you will know both the answer and the answerer, and that they are in a way, one in the same.
(If this all seems vague, don't scorn
I'll explain it when I'm born)
Since I reckon that I have the answer, and since I've now answered with it, I can tell you that the answer is the answerer. Straightforward, no?