Hey, one question at a time. Add a seperate question for the sun script. To add colour to your terrain simply go into inspector for your terrain and click the paint terrain button. Then add some textures and click on your terrain to paint it. To add trees create a tree prefab then go into inspector for your terrain and click on the tree creator button. Add your tree prefab and just click to place trees. Try looking here for more info: http://unity3d.com/unity/features/terrains One note, maybe if you don't know how to script or read a help file you shouldn't be making a game...

Good Luck!

I would recommend the flare pack available on Unity's website if you're looking to create a sun. Apply the sun flare as a material on a point or directional light. If you're referring to a script for a working sun that travels across the sky and alters time of day, there's a series of video tutorials on the subject available on http://burgzergarcade.com that explain it thoroughly.