Take advantage of every spare moment. If you can't spend an hour writing, write for 10 or 20 minutes. Being a successful writer is more about developing good writing habits than anything else. J.K. Rowling would "dash to the nearest cafe and write like mad" as soon as her young daughter fell asleep. She wrote "nearly every evening." Try to develop a similar habit.
Whether you're writing fiction or nonfiction, make sure the material or topic is something you're passionate about for two reasons. First, if you're not writing about something you love, you may find yourself struggling to write. Second, if your work is published, it'll be associated with you for years. Make sure you want your subject and your name mentioned in the same sentence. Plus, as Rowling says, "stories are better if they are rooted in your feelings."
Don't feel too discouraged if your manuscript is rejected. Even if you have an agent, finding the right publisher can take years. Rowling certainly wasn't published overnight: "It took a year for my agent, Christopher, to find a publisher. Lots of them turned it down." Persevere and continue to submit your manuscript.
Rowling has written "quite a lot" besides Harry Potter, including "short stories, bits of novels for adults, all kinds of things." None of it has been published. Remember that not everything you write will be published, for whatever reason. Don't view this as a negative. Write for love of the craft, and view your unpublished material as practice for your soon-to-be-published material.
Writing is often more difficult for published writers, "because there are expectations to live up to," Rowling says. Imagine how she felt while working on a manuscript that was submitted two months late: "The whole profile of the books got so much higher since the third one and there was an edge of external pressure." Understand that you'll likely encounter the same pressure and that others will, too. This can help you work through it and keep sight of your goal.