XML : How to implement ids.xml file as it shown in Google Developers guide?

I am trying to upload my libGDX game to google play store but although it just took mine 3 days to develop the game I am unable to setup google play services almost for one week now. First of all please excuse my ignorance but I just don't understand this modify your code section. Don't think I didn't search for it yet again I couldn't understand these:

1) Open res/values/ids.xml and replace the placeholder IDs. If you are creating an Android game from scratch, you will need to create this file first.

a) Specify your application ID in the app_id resource.

b) Specify each achievement ID that you created earlier in the corresponding achievement_* resource.

c) Specify each leaderboard ID that you created earlier in the corresponding leaderboard_* resource.

2) Open AndroidManifest.xml and enter your package name in the package attribute of the element. If you are creating an Android game from scratch, make sure that you also add the following code inside the element:

   <meta-data android:name="com.google.android.gms.games.APP_ID"          android:value="@string/app_id" />     <meta-data android:name="com.google.android.gms.version"         android:value="@integer/google_play_services_version"/>    

Firstly, I created ids.xml file however, what is this placeholders id's that I should replace? There is no such thing like that.

Secondly my game doesn't have an ID. What is this app_id resource that I should specify? Where am I going to specify it? Is it something like this <item name="app_id" type="id"/>

Thirdly, game is maybe too simple so that there's no achievement nor leader board. Do I have to add these things that I really don't know at all?

Lastly, android:value="@string/app_id" gives error. Is it because I am unable to create ids.xml file properly or do i have to add something to strings value?

I just started developing apps for Android. My question is probably too easy that I shouldn't even ask it at the first place. But I believe if you help me with this it would be a fantastic guide for people who just started developing games. Any help will be very much appreciated. Many thanks in advance!