Well first you can either buy a DVD version of rFactor or download and pay it from http://www.rfactor.net/
If you choose to buy the online version, you'll receive an activation code via email, you'll have to enter it when you'll launch the game for the first time to activate it (that's how I bought it).
If you prefer the DVD version, you'll only need to install a small patch to be able to race online.
Make sure your PC is strong enough for rFactor, as it's a bit more demanding than NR2k3.
We almost use only mods and add on tracks as rFactor itself is quite boring. They are quite easy to install, most of the time it's just a rar file to extract in your rF installation. You can download almost all existing mods and tracks here :
http://www.rfactorcentral.com/
Coming from NR2k3 I think you will find the feeling of rFactor really different, some like it some don't.
We have some american and canadian guys racing here, no problem of ping or whatsoever but of course the timezone difference can be a problem.