Writing an erotic story doesn't necessarily mean a story filled with nothing but sex, of course it will have a sexy scene, but it could even just arouse the reader by tempting them with the anticipation of sex.  Above all it should turn the reader on.

If you read any good novel or short story there are erotic stories within them; scenes that lead up to sex.  The key in writing a good erotic story is being original, clever, detailed and caring about the characters.  You can start by writing about what you picture when seeing someone you like in your mind.  Picturing her or his body next to yours.  Even how she sits when she crosses her legs, perhaps catching a glimpse of her muscles as

 they tighten on her upper thighs.  Or how she pushes up to you and presses up against you,  you don't react but you knowingly let her feel comfortable as she leans closer to you and then you notice how she smells and this turns you on even more.  Creating a good erotic story is capturing these fine details that turn you on and creating a story out of them. 

Also, surprise is always a good aspect in any kind of story, so you could be talking to the girl or guy you like at work and she leans up against you while she is showing you how to do something at the computer and then you run into her just as you are leaving work in the elevator, just you and her, both caught off guard for a moment, somewhat nervous, and your story might just take off from this point - you walk her to the subway or stop and have a drink or stop the elevator right there and then.

Although to get to this stage in writing your erotic story, first, you need to give the reader a feel for what the characters are all about.  Describe them in a few sentences, and this is the difficult part, you want to give the reader an idea of what your characters are like in a way that builds interest and curiosity.  This helps to build arousal throughout the story like foreplay does during an intimate moment.

Certainly there are all different styles and various categories of erotic stories, but the best one's are where all the characters are aroused.  So you need to care about your characters and see that the turning on and getting off is reciprocated by all the characters.

Finally, have fun writing these erotic stories, write one for your wife or husband or boyfriend or girlfriend, if you are having fun writing them it will come across in the story it self.  So make people laugh and turn them on with your wit as well as turning them on with a sexy scene.