How can I call fragment class in activity?

How can I call fragment class in activity?

s2.java..
public class S2 extends Activity implements OnClickListener,OnTouchListener
{ ...
@Override
public void onBackPressed() { .... } }
s1.java
public class S1 extends Fragment implements OnClickListener {
... }
now I want to call s1.java which extends fragment in s2.java onbackpress().