Hi,
Most technical writers are writers that started out in a specific industry, then decided to become a writer. For example, a science or medical writer may have started out as a scientist or a doctor. A writer who specializes in writing technical manuals for The Red Cross, may have started out as a volunteer. What industry or industries do you have extensive knowledge in? Start there. Then use your current writing knowledge and take a course in technical writing. These can be found in person and online. Some companies offer a technical degree or certificate that offers specific techniques for technical writing. Search specific universities or community colleges. These schools/organizations will teach you to take technical documents and transform them into something that can be understood by a non-technical audience. Note that technical writing doesn't always mean IT manuals, it can mean writing instructions for how to build an Ikea bookcase. Although you can learn to write in a technical way. there's no replacing in-depth knowledge on a subject that can be be gained by working for a company as a technical writer. Sometimes you have to throw yourself into a job and learn as you go. I wrote for a technical support site for PBS with no experience in technical writing or on the subjects I was writing about, but I interviewed subject matter experts (SMEs) and wrote knowedgeable articles.