I have done some aiding at Cosmic, Piddo and Berowra. Stay on the solid cracks where there are no handholds to brake. I would go to the crag on a weekday so as not to lock up the classic lines on a weekend - just use some common sense.
To start out I would drop a fixed line down a route and then aid the line just pulling the slack through on a gri-gri (and occassionally tieing in), this helps you get your system sorted without being to scared and it also means you can do a lot with a small rack (back cleaning on lead is always an option but not what you want to worry about on your first outings).
I started with a few short lines at berowra - Positive Ape Factor (one blank section requires standing in sub2s and a blind placement), Savage Cabbage (easy and varied) and Soap (great first lead mixed sizes so you dont need a huge rack). Soap is also a good route to clean as it traverses under a roof. Stay off Mental Fatigue the flake moves and you won't be popular if you brake that off.
At piddo I did Flake Crack pitch 2 (ok but you need some big cams) and Eternity (good), there are a heap of other options there, At cosmic I did Interstate 31 and Gentlemens Drag (be careful here I am not sure how this route is since the rockfall). Good luck.