You could start by downloading the free shade-c 0.90 or beta 0.91 from havoc's website. You have several features there that can help you improve your ingame levels.
In my opinion what makes a scenario look photorealistic with the current technology is the combination of lighting shadows and bump maps. Therefore you could read up on high dynamic range lighting (along with bloom effects), realtime ambient occlusion (ssao), and normal/parallax mapping techniques.
Secondarely you can add depth of field to your level (meaning blury backgrounds), although not many players like that (I personally do).
