The only way you'll ever get better at programming is to write code. In general; just ignore the temptation to write GUI apps (Android, Desktop, etc.) -- focus primarily on writing code that runs at the command prompt; the GUI stuff will come later.
As for the certification; download a list of the required topics, and set up a plan to learn this -- as always the only way to become proficient at this, is to write a lot of code that incorporates these topics.
Certification alone isn't going to make you a good programmer; that requires a lot more coding and study:
- Practice typical programming problems: start with basic ones and progressively increase the difficulty)
- Study and recreate common algorithms and data structures
- Study and implement the most common GoF (Gang of Four) Object Oriented Programming(OOP) patterns
- Java 8 has introduced quite a bit of new syntax and techniques stemming from Functional Programming(FP); Study and implement solutions using this.
Note:
The Java 9 and beyond roadmap is focusing quite a bit on adding even more Functional Programming features, so the sooner you become acquainted with this the better (I'd even prioritise this over OOP).