How to start Fragment from an Activity

You can either add or replace fragment in your activity. Create a FrameLayout in activity layout xml file.

Then do this in your activity to add fragment:

FragmentManager manager = getFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.add(R.id.container,YOUR_FRAGMENT_NAME,YOUR_FRAGMENT_STRING_TAG);
transaction.addToBackStack(null);
transaction.commit();

And to replace fragment do this:

FragmentManager manager = getFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.replace(R.id.container,YOUR_FRAGMENT_NAME,YOUR_FRAGMENT_STRING_TAG);
transaction.addToBackStack(null);
transaction.commit();

See Android documentation on adding a fragment to an activity or following related questions on SO:

Difference between add(), replace(), and addToBackStack()

Basic difference between add() and replace() method of Fragment

Difference between add() & replace() with Fragment's lifecycle

Simple way

1. Create a new java class

   public class ActivityName extends FragmentActivity {
   @Override
    public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
       if (savedInstanceState == null){
       getSupportFragmentManager().beginTransaction()
               .add(android.R.id.content, new Fragment_name_which_you_wantto_open()).commit();}
    }
   }
   

2. in your activity where u want to call fragment

    Intent i = new Intent(Currentactivityname.this,ActivityName.class);
    startActivity(i);
   

Another Method

1. Place frame layout in your activity where u want to open fragment

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

2. Paste this code where u want to open fragment

       Fragment mFragment = null;
           mFragment = new Name_of_fragment_which_you_want_to_open();
           FragmentManager fragmentManager = getSupportFragmentManager();
           fragmentManager.beginTransaction()
                   .replace(R.id.frameLayout, mFragment).commit();