When I began Deadworld, it was an open-ended single title book. That is, I wrote it as a complete story, but left things open enough at the end for there to be more if it came to that. I was not intending a series, but I wasn’t going to complain if that’s what it turned in to. Lucky for me, my publisher wanted more.
I had no idea when I wrote the first book, what the next in the series would be. I merely came up with setting and background that would allow for more stuff to happen, but essentially ignored it intially. I stared writing a new story (as all writers should do). This isn’t to say I gave future stories no thought, because I had some ideas for the sorts of things that could happen or possible directions things might go in, but I did not plan any of that into book one.
The major benefit of this is that I had a story that worked on its own. It did not depend on being able to continue. Major threads were all tied off in the end. I believe this is a good way for unpublished writers to go. For one, you have a story you can sell as is. Many a story has likely been turned down because it was the first in a planned X number of books. From a publisher’s standpoint, these are higher risk. It’s also riskier for the author. If that book doesn’t do well enough for the sequel, you alienate those readers who were expecting/hoping for more. This can potentially cause problems for you down the road with other stories. Though not really the author’s fault, readers become less willing to invest if they lost out before.
So, start small. Create a story within a bigger world of possibilities. Hint at what might be to come, but keep your story within the bounds of a single volume. You raise your chances of getting published, I believe (not to say series don’t sell, because they do, I’m just speaking of the odds here). Have characters that still need to grow and develop. Have a world you will want to explore and readers will be eager to explore with you, but don’t make your story dependent upon getting there, at least not when you’re starting out.