You need to add the right intent filters for your activity in the Manifest xml file.
If 'MainActivity' is the name of your activity that you want to start first when the app opens, add the following to the manifest file. Remember that you have these (i.e. DEFAULT & LAUNCHER) category fields in the intent-filter only for a single activity in your application.
<activity
android:name="com.example.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>