If you use a Android Smartphone (other Mobile-OS same) you should take a minute to get back your privacy and security!
This points helps to prevent, but there is no warranty for 100% protection!
First every Smartphone offers a Factory Reset, search it on Settings and DO it! This prevent you from Firmware Spam by the Reseller! (Samsung, HTC..)
Boot the Phone without inserting a SIM card, create a fake account to get Updates and Software over a PUBLIC WIFI Network (Coffee Shop, Freifunk)
Install wanted Apps, then go to Settings now to accounts, PURGE the fake account.
Disable / Purge Apps on the Settings, of Apps that you never will use.
Check on Settings which App is running by timer, if not needed disable it.
Check Webbackup Apps and disable or prevent them from calling home, (Default builtin Android Firewall)
DON’T setup your REAL Email Account which is used for Home Banking or Online Shopping, CAUSE the OS is open like a Swiss Cheese with holes! (Less Updates by Manufacterer) or to PROTECT your phone from being hijacked by a E-Mail Virus
Encrypt the Phone, and SET Screenlock with Password longer than 8 Digits, same on SIM Unlock Code!
Backup the Smartphone by USB Cable Monthly to a full encrypted PC!
If you are not a newbie, look for Browsers like icecat-apk on the Open Source F-Droid Shop which are more secure
Try Tor Browser later! For more Security!
DON’T Enter words, names, numbers who are CONFIDENTIAL! Cause Smartphones are like papers on a public table without real protection!
Try to use the „Quick Switches“ of the „Screen Pull Down Menu“ of the Home Screen where Data, Autosync and Flightmode can be used.
If you use Messenger Apps check their Settings to disable Download of „Video, Images, Files“ this can be a Backdoor too!
Disable „Video Autoplay“ on Apps like G+ or Facebook.
Use every App with a own fake Account.
NEVER leave your Phone on a public table, cause the USB Port is always open and AUTO Connect every plugged in Cable!
Put a black Strip on the Camera lens if you don’t need them! especially the Front Camera!
Call your Phone Provider, to disable the transmission of MMS/SMS! This prevent Messages with bad links.