The three most common mistakes made by writers are: (1) Writing too slowly on the first draft. On the first draft, do not stop to edit. Go with the flow of ideas. (2) Not knowing the template. Virtually everything that has been written for public consumption has an underlying template. Know it. (3) Not getting feedback from a second or third pair of eyes before submitting something. It is impossible to proofread accuately on your own.
If you want to be a professional writer, it's very easy. Sit down in a comfortable chair in front of a reliable computer. Get up 200,000 words later. You will have your own voice, be able to type quickly, and now be ready to publish.