Hello friend! I started from nothing like you and I have been working for 4 years now Cryengine is an amazing engine and can without a doubt help you achieve your vision however there are many steps you must take to get to a solid point for creating your GTA 5 like game I'll just tell you what I did to get where I am at now. I recommend you start with learning the basics here are a few sites that can get you on your feet keep in mind that software is expensive so until your ready to develop commercially it is best to get non-commercial or free licenses to whatever software you can.
" I learned alot from Pluralsight I definitely recommend it for a beginner furthermore it isn't to expensive.
" This is a great starting point and not to expensive however I have found some of there tutorials particularly Cryengine tutorials to be very outdated.
" Honestly I didn't get much out of Udemy due to the site not really being focused on game design and you never really know what your going to get out of it. However it is an option.
" This is an amazing school I learned most of what I know from CGMA its more money and will be alot of work but it is worth it!
" The online portion of Think Tank is what I am enrolled in currently It is a very hard very demanding school don't expect to be able to hold a full time job while doing it furthermore it is very expensive for me it came out to around 2500 US dollars however you will learn alot and all you need to know without a doubt.
Personally I would suggest using Pluralsight as a starting point to get used to the software you want to use and make your decisions on what kind of workflow you want to have whether you want to use Houdini, Modo, Maya, 3Ds max or others for modelling. Don't let anything discourage you its alot of work but can be done if you work hard and stay focused! Good luck mate!