In order to improve your proficiency in any language, you'll have to work on "comprehension" and "expressing yourself" in that language. Comprehension includes reading and listening while expressing onself includes writing and speaking.
In my opinion the crucial point in learning a language is to find a way to make learning this language a positive experience. So my advice would be to find something that you enjoy doing, e.g. listening to music, reading a book, watching videos on youtube etc. Maybe even write a diary in German if this is something you like to do. Finally you'll have to practise speaking, so you can visit the country, do a tandem language exchange, or take lesson with a native speaker, whatever suits you best.
I hope this is helpful to you!