To ensure you don't inadvertently install malware through the installation of apps outside of Google Play, open your Settings menu, look for the Security option, then ensure the option for Unknown Sources (allow installation of apps from unknown sources) is disabled.
If you're determined to install an app from outside Google Play:
You can install an antivirus app, and plenty of free Android antivirus apps are available that are able to detect and remove malicious apps, for example 360 Mobile Security, Avast and Lookout. These all include an app scanner that will seek out anything dodgy, but note that these apps can also trigger false-positives - reporting an app you've been using for months as malware when you know it's fine. In most cases you can simply ignore these alerts. Also, again with Gunpoder in mind, these apps may not pick up on malicious behaviour if the app is able to hide it from them - Gunpoder uses the Airpush adware library to conceal that behaviour.
Source:
http://www.pcadvisor.co.uk/how-to/google-android/how-remove-virus-from-android-phone-or-tablet-image-gunpoder-3605390/
