For AviSynth, you need to know about only two places: AviSynth.org (http://www.avisynth.org), and Doom9's forum (http://forum.doom9.org).
AviSynth.org is an online Wiki manual/user's guide, and is pretty comprehensive. Doom9's forum is where those hardcore videophiles who actually write filters and scripts and functions go. You will not need any other places to start.
VirtualDub has a new release today (1.6.8 or something?); you can also get info about how to use VDub at Doom9. I think a lot of us more experienced video tweakers use VirtualDubMod in conjunction with AviSynth. That's a very powerful, flexible, and totally free setup.
One thing to keep in mind: always use the smallest number of filters you can get away with. Using too many makes things slow, harder to troubleshoot, and (often) crappier.
Have fun!