We've all done it, as we read a story on world famous Literotica we think, 'I can do better than that.' Well why not, maybe you can write a better story but you'll never find out unless you try. You should have a word processing program such as Word, or Works, etc. This helps a lot as it has a built in spell check and will even help to correct English mistakes.
I would think the easiest way to start is to take the story you just said you could write better, now copy and paste into your word processing program. Go through and make some changes to where the tale fits your thoughts on how it should be told. Now read the original and then your version, how'd you like it, was it really better, or just different? OK let's say you thought it came out better, well that's good except now you have plagiarized some hard working artist's story, and that's a sin my friend.
Next let's try a story from your imagination, don't tell me you can't because we all have fantasies we just need to turn them into stories. How do you do that, all it takes is practice. Start by writing something small, not a story, but a scene. Get a picture in your mind now describe that picture as clear as you can. Now read your scene, does it accurately display your mental picture in words? If not make some changes now reread, don't be discouraged if it doesn't work, this is not as easy as it looks and it will take some practice.
If you can't find the words to describe the picture in your mind, then try to fit the picture to your words. If this new mental image is not as interesting as your original, then go back to your words and try to find what missing. Let's say your mental picture is of a woman, that's always a good image to have in your mind. In your mind, she is sexy with large breasts and long flowing hair with sensuous lips. Once you have put this down it describes a woman but still leaves out details of your woman. We now know her breasts are large but how about her nipples, are they erect and large or small but protruding nubs. How about her eyes, are they large and innocent or dark and mysterious. As this is just a scene, you should go into more detail than you might in a story. First, picture her in your mind and describe her in as much detail as you can.