read file from external storage

You can just do this:

File f = new File(Environment.getExternalStorageDirectory()
                            .getAbsolutePath() + File.separator + fileName);

Better still:

final File file = new File(Environment.getExternalStorageDirectory(), filename);

Better than using File.separator is this, which is standard Java:

final File file = new File(Environment.getExternalStorageDirectory()
                          .getAbsolutePath(), filename);