1. LOD is automatic stuff stored in DirectX. You have to make 4 different models with _01, _02...etc at the end of entity's name to show the engine which one should be loaded.
2. grass... you may duplicate it in script f.e. while number of grass is less than something then make another grass at my position plus random distance. Like my.x += random(2000) -1000; And read about shaders on Wiki
3. Trees... is a lot of different ways to make trees. You may duplicate dummy model with function to create tree at his position. You may duplicate them like a grass. You may try to search some terrain (map) generator to put the trees. And of'course use LOD and fog settings to keep playable framerate.
4. Sun, sky, clouds...etc just READ HELP(F1) time to time! and search forum to find all Your answers.