Before you begin level design on your computer, you should sketch the level layout—a draft of what the full design may look like. For example, we can sketch a simple level for a platformer game; this example is based on a jungle with many rocky platforms. The player is going to be picking coins and diamonds and also avoiding antagonists.
Then, with whatever computer graphics tool available, you can transfer your sketch to the computer and color it into a more attractive design. (The current design is based on the open-source platformer game assets provided at http://opengameart.org/content/gfxlib-fuzed).
And the following screenshot shows what my final game should look like with a few edits, coins, diamonds, and antagonists: