Hi, if the google play store is installed in your device that is the best way to install play store apps. Installing apps from a browser such as Opera Mini is not recommended. If you don't have the google play store app, you can follow these steps to download and install it:
Enable the checkbox at Settings -> Security -> Unknown Sources, don't worry about any warnings unless you are not sure about the safety of something you are installing.
Open a browser on the device you need to install Play Store on and go to this link: http://www.apkmirror.com/apk/google-inc/google-play-store/
, you will have to wait 10 seconds before the Download button works
Download the latest version unless you have an older android device, and use any file manager to install the file by simply opening it and following the instructions, disregard any warnings. It should be in the .apk format.
**Note: If you have an Android OS before 2.3, you will need to choose an older version and check to make sure it's compatible. For 2.2 refer to this link: http://www.apkmirror.com/apk/google-inc/google-play-store/google-play-store-3-7-11-android-apk-download/
Simply open the Play Store and follow onscreen instructions to login and start installing those apps!