UI components that can be embedded inside your activity should be derived from Fragment rather than Activity. When converting your child activities into fragments, you will need to override onCreateView instead of onCreate in order to load the fragment's layout.
In your main activity's layout, you can directly insert the fragment you want to show initially, and give that fragment an ID. Then you can use code to replace the fragment with that ID with a different fragment.
This is a good place to get started: http://developer.android.com/guide/components/fragments.html
This is too big a topic for me to cover everything - you really should look at the Android developer resources - but here are some examples.
As I said, you can put the initial fragment directly into your activity layout. "Fragment" is located on the "Layouts" tab of the layout editor. You give that fragment placeholder an "Id" you can use to identify it as well as the "Name" of the fragment class that will be there to start with.
Then when it's time to switch the fragment, you can use code like this:
// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();
// Replace whatever is in the fragment_container view with this fragment
transaction.replace(R.id.fragment_container, newFragment);
// Commit the transaction
transaction.commit();