Navigation drawer default fragment

I am novice developer and I´m integrating Navigation drawer in my app with android-support v7 and I have one question. When I start the app the main layout is this:

<?xml version="1.0" encoding="utf-8"?>

<!-- The main content view -->

<FrameLayout
    android:id="@+id/content_frame"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<!-- The navigation drawer -->

<ListView
    android:id="@+id/left_drawer"
    android:layout_width="240dp"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:background="@android:color/white"
    android:choiceMode="singleChoice"
    android:divider="@android:color/transparent"
    android:dividerHeight="0dp" />

and this is my main activity:

drawerList.setOnItemClickListener(new OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                int position, long id) {

            Fragment fragment = null;

            switch (position) {
                case 0:
                    fragment = new Fragment1();
                    break;
                case 1:
                    fragment = new Fragment2();
                    break;
                case 2:
                    fragment = new Fragment3();
                    break;  
                case 3:
                    fragment = new Fragment4();
                    break;
            }

            FragmentManager fragmentManager = 
                    getSupportFragmentManager();

            fragmentManager.beginTransaction()
                    .replace(R.id.content_frame, fragment)
                    .commit();

            drawerList.setItemChecked(position, true);

            tituloSeccion = opcionesMenu[position];
            getSupportActionBar().setTitle(tituloSeccion);

            drawerLayout.closeDrawer(drawerList);
        }
    });

How can I set default fragment like main layout of the app? Thank you