Blankets Creek is the closest to Marietta. look here for directions, maps, etc:
http://www.sorbawoodstock.org/ (about 10 miles of trails there)
If you want a longer ride with more climbing, etc. head about 1.5 hours north to Bear Creek, Bull Mountain, Stanley Gap, etc.
here are some resources for you:
Southern Off Road Bicycle Association -
http://www.sorba.org/ - drop this question on the forum over there and you might even find someone willing to show you around.
The SORBA Roswell/Alpharetta chapter has a good list of trails with maps:
http://www.rambo-mtb.org/ (click trails)
There is good riding here, enjoy!