First of all, congratulations for deciding to be a competent writer, this a great challenge of life. And, even I too am an aspiring writer myself, I'm 22. I've developed my writing skills in the last 3 years from the scratch. So, according to me, writing Blogs would be of immense help, rather than directly writing stories, short stories. Firstly, practice writing articles/blogs on the topic that interests you or the concept that you chose to write a short story about. Post the blog link over here or on your facebook and get it reviewed by others who would give you true suggestions. Writing on twitter also helps a lot in developing short lines or sentences, or for telling something specific in just a small sentence of 140 characters. I do it very often (
www.twitter.com/krishna_mydream) it helped me so much to develop the briefing of sentences. The most important of all, is YOU SHOULD BE IMAGINATIVE AND KNOWLEDGABLE. You
MUST read a lot, a hell lot, read very varied stuff, because we never know how and what kind of ideas will be generated while reading what kind of stuff. So, don't afford to miss a chance. And, above all,
never ever let the Spark of your passion for writing and becoming the kind of writer you want to be put-off.

All the very best
