I second the spirit of what Craven said. But since you write short stories, an alternative to agents might be that you find literary magazines that publish the kind of short short story you like to write, and submit short stories directly to them.
Nielsen Hayden, an editor,
has good advice about the formalities of submitting manuscripts. He has especially good advice about what
not to do to avoid being rejected, and how
not to react when you are.
As for the craft of writing a text, the best online references I know are William Strunk:
The Elements of Style and George Orwell:
Politics and the English language. Orwell's text has nothing to do with short stories or even with fiction. Nevertheless, his general advice should serve your purposes well.