Setting up a mailbox for a 3660

Hello,
I just got of the phone with a Nokia rep and I think the person was a
little bit confused and would like if someone might be able to help me out
what I would like. Ok, lets say I send a text message using my cell phone
no. 0000000000@tmomail.net (example only). Now when I receive the message I
will see in the upper left hand corner a envelope and a message saying show
and I can open it and read it etc. Now from what the rep told me is that it
will go automatically to my my mailbox but how can it go to my mailbox if I
don't have any of the setting setup? From what I can fiqure out is that I
can not setup a mailbox without have a internet account with tmobile is this
correct?
If I can setup my mailbox can someone tell me what setups to use for my
tmobile?
thanks