Updated: Dec 16, 2019
If you haven't already learned, one of the biggest things you will need to know is how to properly code your game to work. Even though you have all of the characters and environments put together and ready to go, you will still need that logic behind them to allow everything to interact with one another properly.
The first thing I did was go on the search engine and type of "free computer programming PDF" as a place to start. Just doing that will get you started with large electronic books to download and read through. There are books for all different types of languages, and some are even geared towards learning it utilizing a game engine.
I think I have about 5 books downloaded to read through myself. Some are more beginner territory, some advanced, and some specific to the Unity Engine. Although I haven't spent near as much time reading through them and testing the code (seriously, you need to spend a good 2-3 hours a day on just this) I am able to work through and manipulate code and have a general understanding of what it is trying to do.
Another learning tool I have tried was Plural Sight, an online video training course database for just about everything you need. I first came across it for work when I needed training to further my understanding of BIM (Building Information Modeling) and utilizing my software more efficiently. While there I noticed that there were multiple large courses specifically for programming languages and game engines.
The main game engine courses are for Unity, Unreal Engine, and even Stencyl. Along with training videos, there is also downloadable content to use and follow along with the videos. I found this really helpful as sometimes following along in a book can be difficult where watching a video and seeing what and where they were typing or clicking was easier to follow in some cases.
Another source is Udemy, another database for online training courses. The difference between them is Plural Sight is a yearly membership with unlimited access while you have an active membership, where Udemy is a buy once and have forever system. You can buy a course once and use it as long as you want.
Whether you prefer books or online videos all depends on your learning style. You can even find specific training courses on YouTube.