You have to ask yourself what it is you want to do:
a) want to create a fully playable game people can download and play?
b) want to create a small chunk of game, like a demo or proof of concept; for learning purposes and to get a good idea of what you can pull off and how long it takes to finish something like that
Based on what you have said so far, I would recommend option B. Don’t bite off more than you can chew, because what will happen is that you will loose momentum as you realize how difficult this undertaking actually is as the weeks go on, you will loose motivation and never finish anything- that’s a habit you don’t want to get into. Does that make sense?
The idea is to set yourself up for success and not failure, which is what I think most people do.
Don’t stop from keeping your super ambitious great game idea written down somewhere, you can always come back to it later. You’re young as hell, which means you have lots of time to improve and learn more dev skills, which you will eventually use to create your masterpiece. For now, take a tiny piece of your game and make a prototype of that, only focus on that. Polish it and improve it until you have something really cool. By then you will have a better idea of how much time and work is required to create a game, which will help you scope and budget your future projects much better. Again, setting yourself up for success.
Finally, I have to repeat this because it’s so important, don’t work on any characters or final art yet. That comes last. First work on getting your game mechanics and systems working. Blueprint is great for this because it allows for incredibly rapid prototyping. Use placeholder art for now, for example use the default character that comes with UE4 as a placeholder for your main character. If you need NPC’s or other players, simply change the color of the material instance by making red characters with a single click. You can even get free assets from the marketplace, like the Mixamo character pack which brings a ton of free characters with their own animations. Look on the marketplace, there’s more than enough free stuff for you to use. Need a gun? Grab the assault rifle from the Shootergame Example which is also free from Epic! When prototyping you shouldn’t find yourself creating any assets except for an extremely rare case when you can’t find any placeholder assets (i…e your game absolutely requires a giant spider mermaid, there’s nothing on the market place for that 
) Need particle effects? use the ones that come with the free starter pack, there some smoke, explosion, and sparks stuff in there.
Hopefully this all makes sense; work smart not hard. Good luck with your game idea!