From BlenderWiki
Rendering
Mode: All modes
Panel: Render (Scene context, Render sub-context)
Hotkey: F12
Menu: Render » Render Current Frame
Description
This section will give you only a quick overview of what you’ll need in order to render your scene. You’ll find a detailed description of all options in the rendering chapter.
Options
The render settings are in the Scene context, Render sub-context (Rendering options in the Render sub-context.), which is reached by clicking the 
, or by pressing F10.
The Format panel controls the format of the render. The full size (number of pixels horizontally and vertically) and file format of the image to be created are picked here. You can set the size using the SizeX and SizeY buttons. Clicking the selection box just below the size buttons opens a menu with all available output formats for images and animations, which is currently Jpeg in (Rendering options in the Render sub-context).
Now that the settings are complete, the scene may be rendered by hitting the RENDER button in the Render panel, or by pressing F12. Depending on the complexity of the scene, this usually takes between a few seconds and several minutes, and the progress is displayed in a separate window. If the scene contains an animation, only the current frame is rendered (to render the whole animation, see the “Rendering Animations” page).
If you don’t see anything in the rendered view, make sure your scene is constructed properly. Does it have lighting? Is the camera positioned correctly, and does it point in the right direction? Are all the layers you want to render visible? Make sure Blender Internal is chosen in the drop-down list below the RENDER button.
Saving to disk
A rendered image is not automatically saved to disk. If you are satisfied with the rendering, you may save it by pressing F3 and using the save dialog as described in the file operations page. The image is saved in the format you selected previously in the Format panel.
Hints
Click the Extensions button in the Output panel, so that Blender will add the type extension (here, “.jpg”) automatically to image files!
