how to become a good content writer?

Your question is way too general to afford you a useful response except by coincidence. Why was your writing rejected? Grammar issues? Inappropriate tone? Duplicate content? If you provide us with more information you'll be able to get a more helpful reply.

 

Write and write. Practice.
Take a English/Writing class. Read some pages online.

Or you can head to fiverr or outsource.

 

Know the subject you are writing about, forwards and backwards up and down a million times. With the know how will come many great articles. You will be able to write them off the bat.

You will get to where you can open up an article window and write one on the fly, any time. The knowledge will not let you down.

 

^^ Research.
Again, some writing classes would help. Or find another job.