There's no need to change format when submitting on-line. Prepare your manuscript using industry standards just as you normally would when submitting through the mail. These formatting quirks aren't just for fun; they make the manuscript clear and readable and help editors quickly find information. Knowing and using these standards also signals that you're professional about your writing.
So, what is the standard manuscript format? Prose manuscripts should be double-spaced with margins of an inch to an inch and a quarter. Don't get fancy with the font. Stick with something common, like Times. The first page of the story should include your name, contact information, and the title. (That's the short answer. For more details on the particulars of preparing submissions, see the past column on manuscript formatting.)
Of course, if a journal's guidelines stipulate different expectations, make sure to follow them. Standard rules always come second to any special directions from the journal itself.