How many times have you began writing a poem or story, then looked back before you are finished and decided it "stunk"? How many times has this resulted in you throwing your work away? And how many times have you regretted it later?
It has happened to me many times in the past, but I don't plan for it to happen again. Why? Because I've learned some new rules for writing, and at least for me, they work.
Rule #1: don't read the work until you are done. Anything you write is more likely to sound horrible if you are not finished with it, because it isn't quite complete.
Rule #2: Revise, revise, then stop. Check for major errors, such as confusion-of-plot and information that doesn't work with the rest of the story. Then check your wording to make it sound the best it can. Then STOP. Put it away for a day or two. Whatever you do, never revise more than absolutely necessary after the first time. The more time you spend on something, the more bored you will get.
Rule #3: Never, never, NEVER throw anything away! If you don't like it, file it. Someday when you are older, you will find it again and either say "Wow, this is great! Just needs some polishing!" or "Wow, this just gave me a great idea for something else!" Both statements are very rewarding.
I suggest keeping a special notebook just for writing down your ideas, no matter how absurd they may seem when you first think of them. Even if you never use them, some discouraged writer of a distant generation, deprived of ideas, may stumble upon your treasury of creativity and say "Wow..."