practice, practice, practice.
Build an ugly level, make it run.
Experiment with lights (static + dynamic)
Read the forum to learn how others do it.
Lern to create your own textures, that fit to light and mood.
Read other formums to learn how others do it.
Learn a good modelling program and fill your level with top notch models.
Learn to manage materials
We all went through this process.
Your first 5-10 Levels will look awfully. But then you will make a good one.
You will post a screenshot here and some people will ask
"How did he do this?"
