The Android platform comes with a lot of default icons. Check this visual index of the android drawables to have an idea of what can be used.

Some examples of using these icons:

For use on an ImageButton, defined in xml:

<ImageButton 
   android:id="@+id/lock" 
   android:src="@android:drawable/ic_lock_lock"
   android:layout_width="wrap_content" 
   android:layout_height="wrap_content" 
/>

Or the same set in code:

ImageButton btn = (ImageButton)findViewById(R.id.lock);
btn.setImageResource(android.R.drawable.ic_lock_lock);

And another example of using these icons in your menu:

/**
 * Creates the menu items
 */
public boolean onCreateOptionsMenu(Menu menu) {
  menu.add(0, MENU_HELP, 0, R.string.help).setIcon(
    android.R.drawable.ic_menu_help);
  return true;
}