I get e-mails requesting my advice on writing for periodicals from time to time. I'm sure other people get many more requests than I do. But the best advice I can giving on writing and taking input on your writing can be summarized in previous posts from Ted Neward (from a post on reviewing books) and the widely linked to series from Mike Gunderloy entitled "Advice for Writers". Take a look at what these guys have to say and you learn everything you need to know to get started.