Nice start for a first scene. If I build a scene, I first concentrate on the terrain, and possibly add the most important objects (like you did here), but leave out the sky for the time being. Testing goes faster that way.
I suggest you search for 'fake grass' setups, which will be good for the mid and far grassy areas. Then add some populations of (free) grasses (and flowers) in the front. Lots of free grass to be found, here and on NWDA.
You can also mix fake grass (which actually are one or two tiny (0.005) and elongated (8, or so) fake stones with a mix of greenish colors) with more rocky or sandy soil (clipfiles to be found here too). You can use a merge shader with a fractal as mixer, or use highest raise, or add a distribution shader for additional slope or altitude constraints.