Make sure your phone screen lock is on, because whenever you leave your phone may be on desk for a minute, it could be someone access your phone and leaked important information.
Always secure your phone with strong password.
Ensure that your device is locked automatically if phone is idle for some time.
Install security application
Download apps from trusted sources like if you are using android phone then use only play store to download specific apps.
Check your apps permission because many apps require basic default permission like read SMS, GPS location or but something like a calculator that needs network access or an alarm clock that wants to read your contact database should be treated with extreme caution!
Bluetooth and discovered of other device option enable when you need or want to share data.
Update your apps time to time whatever you are using iOS, Android, windows phone, most of manufactured of apps enable the feature of auto update, so make sure that its a checked so your phone automatically update their apps.
Be careful to click on unknown link, or advertisements because its may be harmful for you, through this click, download unwanted apps and install automatically.