I will admit that early on in my writing career, I wrote Twilight fanfic. No, I will not subject you to the tragedy that were those stories. But I can share with you why, despite the horror, this was a great start to my career.
I personally believe that fan fiction is a great start for anyone who is interested in being a serious writer. Why? Well, you can learn how to move a story/plot without having having to develop a lot of the details.
The previous author has done a lot of the work for you in terms of character building, world building, general plot, etc. All you have to do is focus on your own spin-off ideas and the writing.
Personally, it was a great way to motivate writing. I liked being able to manipulate characters that I was familiar with to different situations I wanted to see them in. While I personally didn’t focus so much on the logistics of writing (grammar, continuity, etc.), those are things that can easily be focused on instead of working out all the elaborate plot specifics.
From there I felt comfortable enough with my own writing voice that I started my own stories. Since I have grown into a writer that can generate my own plot that is extensive enough for a lengthly novel.
I honestly attribute my strong writing voice to starting with fanfic. I was able to solidify my style because I wasn’t focused on other elements of my writing.
That being said, if you have any desire to be a serious writer and don’t know where to start, I highly recommend fanfic for whatever you are interested in at the time.