If you want to be a writer, read! Why people who don’t read books still want to write them is more than I can understand.
If you want to be a writer, write what you love and respect your readers. If you look down your nose at genre fiction and are only writing a mystery or a romance because you think that’s an easy way to get published, think again. Readers will know you’re sneering at them, and they’ll stay away in droves.
If you want to be a writer in English, you’d better be sure you’re good at English. The young man who told me “I have wrote a novel” left me too dumbfounded to ask, “In what language?” Yes, it’s possible to write in English even if English is not your native tongue—Joseph Conrad being a prime case in point—but it’s difficult. Query letters and manuscripts that are full of misspellings as well as errors in punctuation and grammar don’t find favor with agents or editors.
If you want to be a writer, be prepared to take criticism. That’s what editors and readers both dish out, and if you think each word in your manuscript is too precious to remove or change, then you’re in for a rough ride.
If you want to be a writer and manage to hook an agent the first or second time out, don’t fire the agent because he or she doesn’t sell your first manuscript. Keep the agent and fire the manuscript because chances are it wasn’t yet ready to see the light of day.
If you want to be a writer and think your job is over once you put the words on paper, think again. Writing is only part of the job. Selling and promoting are other key components. If you luck out and get sent out on a book tour (or send yourself on one), you need to be a capable public speaker so you can advocate for your book. If you are NOT a capable speaker, get thee to Toastmasters and spend a year learning that terribly essential craft.
If you want to be a writer, write! Every day. Published or not, if you have written today you ARE a writer. (Today I qualify, by the way, but answering e-mails doesn’t really count as writing.)