How to make my layout able to scroll down?

Just wrap all that inside a ScrollView:

<?xml version="1.0" encoding="utf-8"?>
<ScrollView
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
    <!-- Here you put the rest of your current view-->
</ScrollView>

As David Hedlund said, ScrollView can contain just one item... so if you had something like this:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
    <!-- bla bla bla-->
</LinearLayout>

You must change it to:

<?xml version="1.0" encoding="utf-8"?>
<ScrollView
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
    <LinearLayout
      android:layout_width="fill_parent"
      android:layout_height="fill_parent">
        <!-- bla bla bla-->
    </LinearLayout>
</ScrollView>

For using scroll view along with Relative layout :

<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:fillViewport="true"> <!--IMPORTANT otherwise backgrnd img. will not fill the whole screen -->

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingBottom="@dimen/activity_vertical_margin"
        android:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin"
        android:background="@drawable/background_image"
    >

    <!-- Bla Bla Bla i.e. Your Textviews/Buttons etc. -->
    </RelativeLayout>
</ScrollView>