wpmu dev team, Ive been a member for far too long messing

Hi wpmu dev team, Ive been a member for far too long messing around with the themes tools ect.. I now have a idea to build a e-commerce site.

Where do I start is my first question? (just a simple question for the basic building blocks)

Thanks
Gee

Ps, this is my very first post since joining...lol