I am thinking of writing a trilogy and was wondering how should I begin. Should I plan out to write three books or just see if the story develops into more than one book? Thanks!
I had an English teacher who was planning on writing a book and he told us that when he took his work in to get published, he found out that it's really up to the publisher if you have a series or not (usually) But he was just saying that it's better to focus on writing one book at a time, and then consider the rest later
I'd start with a three act structure for one book. If you can split each act into three sub-acts - giving you three books that will stand on their own, each with a resolution, and a story arc for the trilogy - you're there. If the sub acts idea doesn't work, you have one big book.
It's not a hard and fast rule, or course - breaking the three act structure might be what sets you apart from the crowd (though this is kind of double-edged and not always a good thing).
If first book looks good and the story looks like developing into more than one book, the question comes up - why stop at three?
End the first book by having the characters solve a problem.
Begin the second book by having the characters discover that they solved the wrong problem, and now things are worse.
End the second book by having the characters solve the actual problem.
Begin the third book by revealing that characters didn't really solve the problem because they didn't address the cause. The problem is now back and worse than ever.
End the third book by having the characters address the root of the problem so that it stays fixed this time.
Now go back and make sure that in addition to the physical journey that the characters make, there is also a personal journey, and that these parallel each other.