First, know what kind of writing is your interest. Do you like academic writing or technical writing that falls to writing with standards or are you freelancer that wants to write on just about everything at any style you like.
Step2
Next, decide on what particular subjects you want to write. This is for you to attain excellence on that area.It is easier for you to improve if you first concentrate on particular subjects and to not jumble your ideas.
Step3
Lastly, read works by other authors. Note their styles. Learn from their works.
Tips & Warnings
If you're only starting to write it is better to stay to at particular subject.
Always give yourself time to improve your work.
Experiment with different styles until you find what you like.
Don't rush. Be patient enough to learn how to write