Can anyone tell me please how far it is to get out there from Sydney town centre and what the best way to get there is?
Thanks.

About 15 k as crow flies I think.
Train to Chatswood then bus. Or bus all the way (from Wynyard in the city, I think)
You can plan your trip here using public transport.

Yeah as Ryb says you can get a bus - try also going to here and put in Route 169 which will give you a few options.

By car, without traffic, it's about 20-30 minutes
By car, with traffic, allow the best part of an hour
Depending on the part of Frenchs Forest to which you're going, it might be better to use
the 169 bus (which only cuts through FF East),
a train to Chatswood + the 136 bus (which has limited stops in FF)
*or a Forest Coachlines bus (private operator that goes EVERYWHERE in FF)

Don't forget the time factor. check the Bus time tables out or otherwise its a Taxi from Chatswood Train station, which is about $25-$30

i live near the CBD and work in Frenchs Forest.
some figures for you based on my experiences:
average trip via public transport in peak hour (169): 60 mins
average trip via car in peak hour (169): 45 mins
average trip via motorbike in peak hour (169): 20 mins
be warned though, the 169 is notoriously late. the return trip to the city the last service for the day at 5.30pm (ridiculous i know), frequently just doesn't show up at all.