What genre do you write? This is a tough question for many writers to answer and yet it is vital to writing a successful genre book. You must know what genre you're writing. If you don't agents and editors will think you haven't done your homework. To be a successful writer one must read. You can't do a marathon if you don't run and you can't write a novel if you don't read. You don't have to strictly read the genre you write but you do need to read a lot of it. That way you'll know what the trends are and who the top authors are. This will come into play later when you're ready to submit.
How do you figure out what your genre is? If there is an element of fantasy, it's fantasy. Such as dragons, faeries, elves, angels, etcetera. Sci-Fi is a completely different genre. Sci-Fi means the novel is based off probability and possibility, technology and advancement in essence. Paranormal falls under fantasy unless we break things down into sub genres which I'm not going to get into now. I'll save that for another post. If there's a mystery to be solved, it's mystery. Pulse pounding action or drama means it's a thriller. If the main theme of the novel is a love story then it's a romance.
You can combine genres but when you do make sure you know what the main genre is. I write young adult, which is a genre in and of itself based around the age of the protagonist. Young adult is my main genre but the secondary one is fantasy. Young adult and middle grade are easy to mix with genres, others are not so easy though. You can have a romantic mystery or a sci-fi thriller, just be sure you know which one is the main genre and keep that your main focus.
For more excellent advice on genre check out this post by Writer's Digest, The Do's and Don'ts of Combining Genres.