To become a content writer You should have the following skills:

Language : The first thing what makes content is language, You should have good command over language to make Your content more lucid to read and attractive, This is something which You will learn over the years By Reading books and Newspapers.

Essay Writing skills : You may have knowledge of Domain but unless You are able to arrange them Properly and express it to other it is worthless, So try to write essays to enhance your skills.

Domain Knowledge : After all, You are writing about something so You should have proper correct knowledge of the domain for which You are writing something.

If you are good at above three things then You can start writing, Nowadays it is easy to write for free and build an audience network, You can start a blog for free on Topic of Your Interest, You can use blogger to make a free blog and then engage the audience with good content.