Re: Samsung galaxy s2 i9100P
Welcome to Android Central! Not all devices are certified for the Google Play Store, although it's odd that a Galaxy S2 wouldn't be. So you don't see the Google Play Store anywhere? If the phone is really old, it might still be called the Android Market.
You could try downloading the apk installer file for the latest Google Play Store and installing it. Make sure you have "Unknown Sources" checked on in Settings>Security. Keep in mind that this may still not work.
You could also try installing the Amazon Appstore by going to amazon.com/amazonappstoreapp on your phone browser and following the directions from there.