A manual will at most tell you what are the specifications of your camera, battery life, screen, ect. The operating system on your phone has changed from when it was first sold to now, so no manual can tell you how to use the software on your phone. Actually I think I am wrong here, your phone probably stayed at 4.4 kitkat, so the manual flybox linked is mostly still true for your phone.
Your phone only has(had) 16gb of internal storage, after 2 years of updates, you probably have 12-13gb of usable storage. I don't know what you will be using your phone for, but if you need more storage, you can expand it by adding any microsd card up to 64gb, anything higher than 64gb wont work. Don't ask me to explain why, this is something I don't fully understand myself, it has to do with when the phone was made, what hardware was used, ect.
This is VERY IMPORTANT if you want your battery to last more than 2 hours. When you are using an app and hit the button to go back to the home screen, you DO NOT fully shut down the app. It will continue to run in the background so it is quicker to load back up and switch between apps. Think of it like tabs on a web browser. If you end up with all your apps running in the background, it will kill your battery and make your phone slow. It would be like have 50 tabs open in Google Chrome on a laptop from 2005. Figure out what button gets you to the screen with all your open apps, and close the apps you don't use every time you use your phone. Contacts app always running, sure, calculator app, close that. I think the way to get to the screen of running apps on your phone would to be at the home screen, then hit the button left of the middle button. Then, you either swipe the apps sideways to close or they will have an "X" to close.
There is no way around it, your phone is going to be slower than a brand new Galaxy S8 and there is nothing you can do to change that. As technology moves, so does software, and it leaves old hardware in the dust. Software is optimized to be fastest and most efficient on new phones with the most computing power, and old phones with less computing power slow down because they can't keep up.
It appears your phone has NFC, so if you want to be fancy and pay at a store using your phone, you can do that. It basically just has your phone tell the register your credit card details instead of the magnetic strip on your card. There are other ways it can work and there are many different apps to do this.
They say if you aren't paying, you are the product. That is 100% true, especially with android. There are loads of free apps on the Play store, there are good ones, and there are bad ones. Best case scenario is you get advertisements at the bottom or top of your screen while using the app, this uses data or wifi, and slightly slows down internet speed. Worst case scenario is the app collects and sells your data to third parties. Luckily there is a way to tell when you might not want an app. The way android works is apps have to ask for permission to use other info from other apps, like a voicemail needs permission to use contact info or phone info. A big red flag is when something like a calculator app asks for permission from contacts, photos, phone, camera, ect. If it asks for permission to things not necessary for the app to function, it is something you don't want on your phone.
