, if you want to write professionally, then go ahead and try. I've used fanfic to hone my skills and fine tune my English, broaden my vocabulary and get ready, like a warm up before a marathon.
I've already written one novel, unpublished, a few years ago and now finally ready to write another one. Once I have my original stuff published I fully intend to go for a Star Trek story and have three plans in development. But those have gone on the backburner while my original-universe stuff takes off.
Good luck, you'll need it. As will I.