Google playstore comes preinstalled on Android devices but I prefer to download an app's apk instead of using playstore because I get to keep the apk for future use and also because playstore can be problematic sometimes. If your internet connection isn't good or your settings aren't right, you'll find playstore very frustrating.
In this brief guide, I'll touch some common playstore issues and how to resolve them.
1. Poor internet connection: If your internet connection is poor then playstore might not even load let alone download apps . Switch to 3G / greater to enhance your internet connection or try later when your network signal is stronger . Go to settings > more > mobile networks > 3G services then enable 3G.
2. Playstore set to auto update: If playstore is set to auto update over data / Wi-Fi, then there might be a lot of strain on your Android and megabytes from playstore as it'll constantly sync to update other apps; thereby, competing with the app you're trying to download. It's better to set playstore to update over Wi-Fi only then revert this wherever you wish to update an app. Launch playstore > menu button > settings > auto-update apps > set to over Wi-Fi only
3. Not logged into your google account on the Android device: You won't be allowed to use playstore without a valid google account. Go to settings > Account > add account > select gmail then supply your details and add.
4. Background data disabled: Playstore won't work if background data is disabled for it. Go to settings > Data usage > select playstore > untick restrict background data
5. Google playstore services disabled: Playstore won't work if background data is disabled for playstore services. Go to settings > Data usage > select google playstore services > untick restrict background data