Don't know when it will start. Jar hasn't set it up yet - he's waiting to see how many players there will be.
As Morris has said, there's a couple of things you need to do first before you can use XSN:
1) Your Gamertag must be "linked" to a .NET Passport. You can do this at
www.xbox.com. If you don't already have a .NET Passport, you can assign one to your existing email address (or get a new Hotmail address) at
www.passport.net.
2) You then need to sign up to
www.xsnsports.com, using the .NET Passport that you have already linked to your Gamertag.
Once the Tourney has started, you'll get an invite on the XSN website, which you will need to accept to enter the tourney. It will also give you a password which you will need to note down.
XSN will pick who you have to play, & by when, & inform you. Arrange with your opponent when you are playing. Go on Live & start an XSN Top Spin game, entering the Tourney code & so on.
The results get updated on the XSN site soon after the Top Spin XSN match has finished.