Rendering is the process of generating an image from a model. This step can be the source of some confusion for new Blender users. If you are having render problems when using Blender consider the following areas of concern.
1. Ray-Tracing or EnvMap must be set in the Render Panel (if in use).
2. If you are getting funny lines across your render you may have accidentally pushed the Field Rendering Button (Render Panel-->Render Section --> Field button).
3. There must be a Lamp and a Camera in any Scene that will be rendered. If there is not a Camera rendering is not possible. If you don’t have a lamp your scene will be too dark.
4. Have you selected the correct layer(s)? If not do so!
5. If you are compositing you must allow the composite rendering. Click the Composite button (under the Render button) in the Render Panel.
Other people have pointed out the following issues related to rendering.
6. If Blender is crashing whenever you press F12 then you may have too many vertices in your scene. Try using the Poly Reduce script when you are in Edit Mode.
7. If you are using an alpha version of Blender then it is very likely that Blender will blow up. Download a more stable version.