The simplest method is to make your app a launcher. Therefore as soon as the device loads or the users presses the home button your app will be shown.
You would need to add the following to your Android Manifest for your main activity that you want to be shown at startup/home key press
<activity android:name=".MainActivity"
...
android:launchMode="singleInstance"
android:stateNotNeeded="true">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.HOME"/>
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
Hope this helps