no offense, but chances are you will have trouble writing ANY story at this time. the reason being, its a new experience that takes time getting used to. as for tips, simply develop a full story ahead of time. thats one of the easiest ways to write a story on here. if its all plotted out, its a simple matter of copying it down on paper/internet.
another reason you MIGHT be havin trouble, research! knowing your subject is key to writing a well developed story. knowing your subject intimately helps tremendously.
finally, try writing a little at a time. dont try to write War and Peace in 20 minutes.
nate
