Where do I start?

I'm developing a product I thought of.
As a part of it, I'm trying to figure out the best architecture for the product.

It's a server application, what is supposed to serve A LOT of users at a time.
Think of an IM application, but not web style, more like ICQ\MSN applications... With web orientation.

I've read all about Meebo, Facebook Chat & etc architectures.
But I'm still not sure where to start on ICQ style (this is the first phase before I go totally web).

Can you direct me to some information on load balancing 10 million users? ;-)
I just don't know here to begin...