The good news is, there is lots of information about modding available for rFactor. The bad news is, it is not conveniently organised in one place, so you need to go out and look for it.
Probably the first choice you need to make is: do you want to create a car or a track? Both typically take many months, so be prepared for that.
The first resource you need to visit is the rFactor Developer's Corner, where you can download some basic tools and documentation to get you started:
http://rfactor.net/web/rf1/devcorner/
You need at least the "mod development tool pack" that also contains some information about how all the different file types work for creating a car. On the same site, there is also a file about getting started with a track.
None of these assume that you still need to learn how to do 3D modelling, so if you do, that is probably the very first thing to start with now. To get 3D models into rFactor, ISI only supports plugins for 3D Studio Max (the plugins for specific versions can be downloaded on that same page). 3D Studio Max is not free, so you need to buy a license. Tutorials on modelling cars and tracks are available and do not necessarily need to be written specifically for rFactor.
For the rest, Google is your friend. There are many sites and forums with information (unfortunately, here at RD there does not seem to be an rFactor 1 specific modding forum, but there is one for rFactor 2) and again a good starting point probably is ISI's own forum about modding:
http://isiforums.net/f/forumdisplay.php/25-rFactor-1-Modding
Hope this helps...
