One of the biggest mistakes new writers make is thinking too much about what sells when they sit down to write.
The worst mistake is to sit down and write something "like" the current best seller. Novices often think that by jumping on the coattails of Harry Potter or Twilight or the Hunger Games they have a solid winner.
Here's the problem: It takes a good amount of time to write and publish a novel. Publishers and agents aren't looking for more of the current fad - they're looking for the next fad. (It's a little different if you're self publishing, as the cycle is a bit shorter). And because so many people do it, inboxes and slush piles become overloaded with whatever the fad is and agents and editors start screaming "Stop!"
There's nothing wrong with considering marketability when deciding what idea to work on next. Writing something you don't enjoy because you think it will sell, though, is likely to produce a low quality book. Following a fad is just going to leave you behind.
Write something you enjoy and make it as good as possible - there's luck involved, of course, but you still have a better chance than by blindly following trends.