Not a bad idea to play in a league first to at least get the feel of what you like and don't like so you can get off and running once you crank up your own league.
I played in a few leagues around for 6-9 months (might've been a year) previous to starting our league and lurked in a WHOLE lot more.
Also, to do it right, in my opinion, set aside a couple of months (less if you don't work) to get the website and system nice and ready to roll once you start signing folks up. Of course, if you're more web proficient than I am (likely), it may go faster. Bottom line is the more prepped you are, the smoother it goes, and the happier your GMs are.
G'luck with whatever you do, and if you haven any specific questions, feel free to hit me with a PM. I'd be happy to help.
-E