So, if you've gone sequentially on this journey then this is about the time that you know your tools and are ready to make your BIG IDEA!
But- it would be wise to not jump into the art and the details until you've built a cohesive foundation . I suggest you take some time to ROUGHLY map out your game's core features and gameplay loop (the main key points of gameplay. Ex: in Mario you reach the end of a level, go to the next, repeat, loop).
Once you have it built, test it yourself, have others test it. Write down issues and feedback. You'll use what you wrote down to build your game's core to be the most fun and the strongest it can be.
For me, I iterated upon my game, Spirit, a lot before I understood the most fun parts. Hopefully the difference between you and I is you don't waste your time making art for each version that you end up scrapping. Seriously! Look at this:
Anyways, I hope this has helped you get started on prototyping!