A few things I can think of:
Remember to add the features and requirements on the manifest for a camera app:
<!-- Camera Requirement -->
<uses-feature android:name="android.hardware.camera" />
<!-- Camera Permissions -->
<uses-permission android:name="android.permission.CAMERA" />
<!-- Features -->
<uses-feature android:name="android.hardware.camera.flash" />
<uses-feature android:name="android.hardware.camera.autofocus" />
If you want to use it as flashlight you can do what @yoah said:
parameters.setFlashMode(Camera.Parameters.FLASH_MODE_TORCH)
If not, to turn flash on which will come out when you take the picture, you use:
parameters.setFlashMode(Camera.Parameters.FLASH_MODE_ON)
I wrote a custom camera app recently, but haven't uploaded it to Github yet, it has all the basic functions working.