That's pretty cool! The horse needs a bump map or something... it looks too smooth. Other than that I think you are off to a great beginning!

The color and value of the horse and his tack are almost identical. Adding a texture map (Daio's are excellent and free) to the horse will help a lot. I would tone down the brightness of the buckle under his neck. The major thing I would do (if this were my work) is to move the sky so that the lighter colored clouds were behind the rider and not the nose of his horse. The eye goes first to the areas of greatest contrast (the old Frazetta trick) and returns often. You probably want people to notice your rider first and his horse second... unless this is a very special horse (Mr Ed, My Friend Flicka, Trigger were special horses). Carolly