How to use a sandbox for new articles
For starting a new article, you may first want to draft the article in a user sandbox named after the topic, such as
User:Stan Lee/Project X,
just as you would when expanding an existing article. When you are ready to make it live on Wikipedia, consider submitting it to the Articles for Creation process first so that experienced editors can check it over. In general, the sooner you move out of a sandbox, the better.
(Articles for Creation often has a considerable backlog, so you should not wait around for a submission to be reviewed. If your submission has not been reviewed, go ahead and create your article once you're sure it meets the basic requirements for a Wikipedia article.)