There is a lot of advice we could give, but think that the most important thing aspiring writers need to hear is writers write. There are rules to learn and to learn to break, there are hundreds of small pieces that go into publishing your work, but at the end of the day, those are skills anyone can be taught. No one can be taught to have stories inside them that demand to be told. If you are one of the afflicted who can't go a weekend without hitting the keyboard or scribbling in your journal, then you are a writer.
If you want to improve your writing or to publish and move from writer to author, then we would suggest reading widely and joining groups with other writers. The are many excellent books on how to go about moving into writing professionally. There are fantastic blogs about the life of a writer, the pitfalls to avoid, and how to publish. There are amazing writing communities online and in many cities that can offer advice and support on your authorial transformation. But, most aspiring writers we meet tell us they want to be a writer "someday." To those people we say if you are writing, if you couldn't stop if you tried, if your brain peppers you with stories every moment it can - then you are already a writer. Welcome to the club.