Writing more is the best method that is there to help you improve your writing skills. You might say, you don't have anything to write about. But that is far from the truth. The internet has several websites that contain lists of writing and speaking topics. Here I will leave you with a couple of websites that have hundreds of topics that you can use to practice writing:
https://www.ereadingworksheets.com/writing/persuasive-essay-topics/
https://letterpile.com/writing/100-Easy-Argumentative-Essay-Topic-Ideas
https://studymoose.com/essay-topics
Set a three month period and try to write a 300 word essay on each of the topics (at least on one topic a day).