First identify what excites you - what do you get obsessed about? What sends your mind into a fit of fantasy? What do you like to talk about? (Or what do you most wish you could find someone to talk to about?) When you identify something of this sort, write it down. Make a list. Keep in mind that this is a brainstorming exercise, so don't censor yourself. Don't tell yourself anything like "No one would be interested in this" or "No one would ever publish this" or "I'd be too embarrassed if my grandma read this" or anything critical of this sort. Just put the ideas down no matter how crazy they may seem. Once you have a list, you can start by telling yourself that your novel will somehow include some or maybe all of these elements. This should get your juices flowing. (One of the secrets of poetry is that the restrictive structure forces you to tap into your creative capacities. So you might consider setting up some fairly absurd, arbitrary rules for yourself – like picking 20 words at random from a dictionary and including them all in your first paragraph, or whatever. You might ultimately discard this paragraph, but jump-starting your creative mind in this fashion can be a great way to get your fingers moving.)
Getting back to your list of obsessions…I once read a novel about a guy obsessed with fantasy baseball (this was back before home computers, so these games were played with cards, game boards, and dice). Now if you think about it critically, this would seem like one of the most boring possible ideas – basically a whole novel about a nerd playing a board game all by himself. But the novel was published and got some pretty good reviews. My point is this: If you are obsessed and fascinated by something, and you write about it (no matter how obscure or quirky it may be), and if you are a reasonably good writer, then there is a reasonable chance that others will find it interesting too.
But the bottom line is to do exactly what Michael Crichton said, you just need to sit down and face the computer screen (or whatever) every day. You will probably have to write 100,000 words before you really figure out what it is you are writing about. That's a lot of work that you end up mostly sending to the trash can, but for the majority of us it is an essential part of the process.
Now the really big question is this: How do I get myself to follow my own advice?