What are the steps to become a professional mangaka comic writer?

You can start writing a comic book right now. Getting it published is another matter. Some people try the D.I.Y. (do it yourself) method. That is exactly what it sounds like.

The other, more difficult, route is to submit your story to a comic publisher. Be prepared for rejection. You may get rejected over 200 times before they accept something and publish it.

Another thing I would suggest is if you are not an artist, hook up with one and collaborate.