It might be a good idea to get a basic bodywork book like Petersen's.

But... start with a wire brush, followed by 80 grit dry sand paper, then 120 grit, then 240.

What you have to do depends on whether or not the metal is rotted through. Simpler if it's not, but you can still do it.

Assuming it's through, one way is to sand away the rust and at least 1/2 in. of surrounding paint. Indent the metal slightly (an eighth or sixteenth of an inch), cut a piece of metal screening to cover the hole and surrounding metal. Put POR-15 on the sanded metal, then put the screen patch on, then apply more POR-15. You may have to hold the patch on with some small screws until the POR-15 hardens, after which you can grind them down with a grinder. After the POR-15 hardens, rough it up with 80 grit sandpaper and apply body putty (bondo) according to directions (get the book!), after it sets up you can begin to sand the bondo down, using progressively finer paper until the edges of the repair are feathered into the surrouding surface. Clean with paint thinner or acetone, spray a coat or two of primer, then a coat or two of the color, again, according to directions (get the book!).

After all that it still may look horrible (or fairly good, depending upon your patience, eye for detail, and basic talent for such things).

Good luck,