Only download and install mobile apps that you can find in the official app stores.
Make sure that the option to allow installation of third party apps is disabled. That means no apps from your online buddies, ads, blogs or torrents. Those usually carry malware that will harm your smartphone.
However, this doesn’t mean that an app or game from the official store is 100% secure – sometimes, even popular apps, with more than 5 million downloads, can prove to be infected.