First things first, answer these questions to decide if writing should be a career or a hobby. Now if you answered yes to these questions, chances are that writing may be a good career choice for you. Now you need to know the most important aspects of the writer's life. You will not sell everything you write, you probably won't even sell most of your ideas. Get used to it and move on to the next idea.
Writers often have to write as a second career, as a spare time passion.
A writer should be able to write on any topic. Read the newspaper, books you normally wouldn't read, or surf the internet for ideas you may not have considered. The majority of money to be made is in the non-fiction market. You will often spend more time researching than writing. Writers must research markets as well as their ideas.