First of all, I would create a custom drawable resource for easy implementation of rounded corners.
(place this in res/drawable)
my_bg.xml:
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle" >
<solid
android:color="ffffff" />
<corners
android:radius="15dp"/>
<stroke
android:width="1dp"
android:color="@android:color/black" />
</shape>
More info on xml drawable resources can be found right here if you want to get into more advanced drawables (gradients, layer-list, animation, etc...)
Then change your TextView in xml layout file to match this:
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="8dp"
android:textColor="#000000"
<!--refer to your custom drawable from earlier-->
android:background="@drawable/my_bg"
<!--center text within TextView-->
android:gravity="center"
android:text="0" />
I hope this helps, Happy Coding!