It's a good start! The cloud height is too high (looks like you used the default 8; try 2 or 1). The streakiness in the shading isn't natural, either (although if you're not looking straight up at the clouds, it will work just fine and look natural). Lower your shading if you still get that after lowering your cloud height. Study real clouds a lot. These are beginning to look pretty real. You can get a download of Terragen cumulus clouds from my free stuff. You can see many images of clouds for study here. Scroll down to the clouds links. This is part of my personal collection of cloud studies. Your sun diameter is too large. :) I like where you are going with this.