To start with, you need some idea of what your level will look like. In a game development environment, you will usually be given this "look" by your art director. As an asset artist or level designer, you will take their specifications and drawings, and turn them into a 3D reality. The level designer will already have thought about game-play, the level of difficulty, the challenges to be contained in the level, overall look and feel, and perhaps the lighting quality and sound. You will also have parameters to stick to within the whole game production:
Is the game set in the real world or fantasy or sci-fi?
What's the date?
Are graphics cartoony, realistic, edgy, dreamlike, happy, or dark?
How quickly should the graphics render: low or high detail?
As well as many more.
Your job as a game artist, level designer, or asset modeler is to stay rigidly within the parameters that you have been given and, having done that, to shine through with your incredible artistic and modeling...