Right now your model consists of *ngons*. In most cases it is better to have *quads* than *ngons* in your model (see this topic- it explains it nicely: http://blog.digitaltutors.com/ngons-triangles-bad/).
To create them, select two vertices on the corner of your model and press <kbd>J</kbd> to join them with edge. Repeat the process in other corners.
 ![enter image description here][1]

Now your model consist of quads. You may add edge loops to your model to sharpen the edges (hover your cursor over model and press <kbd>CTRL</kbd>+<kbd>R</kbd>). See the manual here: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Meshes/Editing/Subdividing/Loop_Subdivide
![enter image description here][2]


  [1]: https://i.sstatic.net/c9VOt.jpg
  [2]: https://i.sstatic.net/YrNuZ.jpg