Have you started writing the book? Or are you about to? When you start, you might find that you have enough words as it is.
Write the story that you have planned so far, even if you're not sure that it's long enough. Write to the end, and then read back over it. You might find that you need to add more descriptions of the characters or of the world around you (adding weather, descriptions of houses and frequent locations, etc).
Also, you may find that something you described in one sentence would actually work as a scene all by itself, or you might figure that instead of always referring to an incident in the past, you could have a prologue or a flashback somewhere that describe that incident.
If you still need a longer word count, you could always flesh out the supporting characters in your story, give them a story of there own (a subplot) - this is what happened incidentally with Jacob Black in Twilight, who was only a passing reference in the first book, but whom the editor/agent liked so much, so Stephenie Meyer developed into a major character.
Good luck with your novel, and remember, although writing is a lot of hard work, it's also fun!