But to answer the question, the most important thing is to write often and focus on your craft. Being a writer is a marathon not a sprint. Don't assume your first (or second or third and on and on) book is going to be a bestseller. Don't even think about book sales. Focus on the words. Focus on how you love to write. Write for the right reasons. Write every single day, even if just for yourself. The more you write, the more you'll learn and the more your craft will improve. Your readers will notice and appreciate that your books get better with each and every one.