Restrict read access on certain folders
Install an app like KeepSafe, which is optimized for pictures, or Super Vault, which you can store different types of files in. They encrypt your data and require a password to unlock, so those will probably be the best choices for you. They don't even require root.

If you want to go a step further, you can even install an app like AppLock to require a password to open a specific app.

Alternative method: You can install a file manager like ES File Explorer and zip a specific folder with a password, so no one can unzip the encrypted data without the password. Keep in mind that you will have to unzip and re-zip every time you want to add/remove/modify a file.