You aren't defining your animation in the correct place nor are you applying it to the text view.
Try this:
public class Pagetwo extends Activity implements OnClickListener {
public Button btn1;
public TextView tw1;
Animation a;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn1 = (Button) findViewById(R.id.BtnKlick);
tw1 = (TextView) findViewById(R.id.tvhallo);
a= AnimationUtils.loadAnimation(this, R.anim.scale);
btn1.setOnClickListener(this);
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
tw1.setText("Hallo");
tw1.startAnimation(a);
}
}