If you're ready, let's try a sex scene, as you can imagine the sex scene is the most important part of a sex story. For your first one you should keep it simple and use a scenario that's familiar with you. The biggest rookie mistake we all make is too many Ahhhs and Ohhhs etc. This might be the way it sounds in your bedroom but it doesn't translate too well on paper. If you feel you need something like this, just describe it as moans and let it go at that. You will soon find out that sex is hard to do well on paper, for basically sex is just 'it goes in and it goes out' and there is only so many ways you can say it. You need to portray excitement for both parties and you do that by the look on their faces or their shortness of breath and the heat of their bodies.
Orgasms need to show satisfaction, again without the two letter guttural sounds we tend to make while climaxing. Just as a public service note here, unless it goes against the story subject we all should practice safe written sex. It is just this world is now full of dangerous STDs and if you can have your characters practice safe sex it will help reinforce people to have safe sex out there in the real world. I know I'm as bad as anybody and I love a good messy cum scene in my stories but I try to sacrifice my art for the sake of the common good.
I know the secret of good writing is practice and as my many critics will tell you, I am not a good writer. Yet I know I have improved and I feel I have evolved into a person who writes stories that I enjoy and if someone else finds pleasure in something I write it's just that much better. I found early on that you couldn't please everybody, so write for yourself, and let the critic's fall where they may.
Literotica has a very talented volunteer editorial section full of volunteer readers who will edit your story free, the one hang up being that most are so busy they take a long time to get back to you. So good luck with your story and if I have encouraged or helped you in any way, send me a note when you story is accepted, I'd love to read it.