Apple have a system that works... but not always how we expect it too!
Some details which may help.
When we first open an Apple acount to buy apps and books etc,. - Thats it... it does not really do anything else.
You say want to log in to iTunes - to upload a song.
What I think is best for you is to look at the top of this page ans you will see "Apple Support Communities"
click on that to open up the full list of Apples varioes support groups, scroll down till you find the iTunes.
Go into that group and - may I suggest - APple do not monitor these groups - so dont waste effort complaining about anything... just make a post to ask for help and information that will help you upload your songs.
My opinion is that you have not actually opened a sellers account, which is a new account and separtae from the normal APple account to buy apss, etc,.but their are experienced sellers whi will be able to help you.