ok so here is a render of my scene (only a basic scene) its all grainy and i dont know why.
ive included the tgd file so you can try and recreate it if you dont recognise the problem
(http://img269.imageshack.us/img269/7255/grainyimage.jpg) (http://img269.imageshack.us/i/grainyimage.jpg/)
Most probably your atmosphere quality is too low.
Try increasing the atmosphere samples to 32. This is a good start, though the green may be too green. I would add a power fractal to the color input. Make the power fractals color an olive drabbish color. It should mix well and allow the bright green to look nice.