Judging from the screenshot I can tell that this map is "fullbright" or "allbright" meaning there are no light entities placed within the map.

What I suggest is to use the entity tool (Lightbulb on the left side of Hammer) and place an entity in your map. Double click it, then change the type to lightenvironment. Change the brightness and colors to your liking. Then use the Pitch Yaw Roll and select Point At. Click where you want your light to go. Using the Pitch Yaw Roll for the lightenvironment entity simulates a sun. Then continue to place basic "light" entities throughout your map. Remember to never leave a light all white! Change up the colors to make the map more realistic!

Another thing you can try is to make your level less blocky. Try to use more complex shapes like cylinders and torus'.

I hope this helps you continue to make maps, and remember to never give up! Your first maps will never be a masterpiece. Everyone has to start from somewhere!

If you have any other questions feel free to shoot me a message.