Truth of the matter is your next step is to just jump in. You will quickly learn what you need to know as you encounter it. Experience is as always, the best teacher.
If you are looking for projects to start with (and what you will learn from them) try this list. If you are happy with Construct and Unity, stick with them, nothing wrong with either engine. If you want to switch, I've done a review of several of the most popular game engines, although switching generally isn't in your best interest. Learning a little about a lot of things can bite you in the ass, a bit of focus is your friend early on.
As to art assets, many people start with kenny.nl or opengameart. The second is a great resource, but I really wish it was curated better. You just end up wasting so much time on garbage, which is frustrating.