One of the most effective ways to deepen your knowledge and skills in any area of endeavor is to pick a project for yourself and then complete that project. It could be a simple project that takes dealer.c, for instance, and uses that to create a Blackjack game or other card game. Or, you might want to create a simple to-do list program using your knowledge of files and structures. It could also be an ambitious project that provides a screen to enter data that will be stored in a database on a remote server.
Regardless of the project, not only will you gain greater knowledge but you will also have learned how to acquire more knowledge to accomplish your project(s). And if you choose to make programming your vocation, you will have working programs to demonstrate your skills.