Most sci-fi and fantasy writers don't write just stand-alone books, but series. Readers, writers and publishers all tend to prefer them. Publishers like them because each book in the series after the first already has a built-in audience. Writers may like the fact that they can re-use the world-building work from the first book in the others in the series. And readers enjoy reading about familiar worlds and/or characters.
Some series aren't planned as series. Marion Zimmer Bradley at first thought it was a sign of laziness when she set a second novel in Darkover rather than inventing a new world. In other cases the author has planned all the books in the series before the first one is even written.
If you have read a lot of series, you may have noticed they come in different types. In order to plan your own series, you will need to know which type yours might be.
- One Big Book series--- like Lord of the Rings, this series comes out in several volumes merely because it is too long to fit easily in one volume.Except for the length is is the same as an ordinary one-volume novel.
- Overarching Story plus smaller stories --- similar to the Harry Potter series. There is an overarching story about the conflict between Harry and You-Know-Who, plus each book has a story which comes to a conclusion by the end of the book. When the overarching story ends, the series is over. But the smaller plots to each book give the reader the feeling that they've read a story which has a conclusion.
- Cliffhanger series--- Each volume has a story which is concluded in the book, but the individual book ends with a cliffhanger--- a dilemma which won't be resolved til the next book. This can be unpopular with readers, however. This type of series is open-ended and can continue as long as writer and readers are interested.
- Episodic series, same main characters--- Think of the Sherlock Holmes stories. The mystery in each story is different and unconnected, but the main characters are always present in each novel. Another open-ended type. In some series of this type, the main character undergoes little change throughout the series. In others, the main character will age, change jobs, move to different cities, marry, have children and grandchildren, and other realistic touches.
- Episodic series, main character group--- This is a variation in which there is a group of main characters. Each novel will have main characters from this group, but different novels will have different main characters, but always drawn from the set. Piers Anthony's Xanth series, at least in the novels I've read, worked that way. The main character in one book was the son of the main character in the first book, for example. In this series form, the main character from one book will turn up as a minor character in another.
- Episodic series, same world/setting--- Marion Zimmer Bradley's Darkover works like this, as does Mercedes Lackey's Velgarth/Valdemar series. The same fantasy and science fictional world is always featured in books in the series, but some books may be set in different time periods with an all-new set of characters. In this type, there may be mini-series within the series--- three books featuring the same characters, for example.
Each of the different series types has its own appeal. As a writer, some may seem better than others to you. For an unpublished writer who is aiming at working with a mainstream royalty publisher only, the first two types may not be a good bet as you will be asking the publisher to stick with you through a whole series--- something he will not want to do if the first book sells poorly. Better to start out with something more open-ended for your first effort. (If you are going the self-publishing route, you can please yourself--- but be sure you are really willing to stick with a story in several volumes even with little reader interest before you commit to types 1 or 2.)
When I started my own series Taliesin, I was actually inspired by a mystery series rather than a fantasy, Anne Perry's William Monk series. That series featured William Monk, an amnesiac police detective, and a woman he meets in the first volume who assists him in his later cases. This gave me the idea of a supernatural fantasy series featuring a vampire and a girl with Dissociative Identity Disorder who together get involved with a series of unfortunate supernatural events in a series of novels.