How can I secure my pictures, videos and other files stored on my phone? I am looking for a way to protect every sort of data that is saved in my Android phone from being stolen. I have done some research on Google Play, the only app I found with all the features is Folder Lock. Other than this app, the majority of the apps do only have a couple of specific features. Is really Folder Lock the most comprehensive app available at the moment?
I would recommend using full disk encryption and keeping your bootlaoder locked and phone unrooted if you want the best possible encryption that Android natively provides.

This article explains how to do a full system encryption on Android.
http://www.howtogeek.com/141953/how-to-encrypt-your-android-phone-and-why-you-might-want-to/