While I will let the judgement of others decide about whether they are "really good websites", some are:
Simviation.com - and the design utilities page, orphan source files page, and this design forum.
Freeflight Design Shop - tutorials, links to tutorials, source files, and and absolutely magnificent design forum (the fact that I host it doesn't affect my opinion in any way, of course) (
http://www.freeflightdesign.com)
gmaxsupport.com - the forums there support gmax, including making airplanes for FS as wells as learning/using gmax.
My recommended approach to gmax is:
Do the tutorials that come with gmax to learn the basics of the program,
then
go to the websites for gmax/flightsim specific tutorials.
Frequent these forums to ask specific questions.
Somewhere along the line, download the FS9 SDKs from the Microsoft site (see my signature), and install, as appropriate, per the instructions.
Thou shalt read and commit to memory such utterings as come from "Milton" and "Firestriker", to name but two sages of the gWax persuasion. Anything uttered, however, in the name of "Inspector Poly" shall be cosnidered with a grain (or more) of salt.