Take a look here:
http://developer.android.com/guide/topics/media/camera.html
There is all the code you need to make an activity which shows the camera preview.
As the Camera API:
http://developer.android.com/reference/android/hardware/Camera.html#setPreviewDisplay%28android.view.SurfaceHolder%29
specifies...you need to use a SurfaceHolder for your preview. Thus you can only use a SurfaceView or a derived class for your automatic preview. Otherwise you can use for example a TextureView or an ImageView to draw your preview by yourself. You'll need though to implement the onFramePreview method to grab the frames from the preview and handle them by yourself. By using OpenGL or the ANI API you can also add effects to your frame. This is the only way, I know about, to also add effects directly on your preview, unless these effects can be achieved by overlaying a semi-transparent colored surface above your preview image.