Do you want the right way or the wrong way? Do you want to do the work yourself? How long do you want the fix to last? You dont have a picture so I am not sure how bad it is. Do you just have surface rust or a hole? The right way if you have a rust hole or weak metal is to cut it out and put a patch pannel in, grind down the welds, light putty, and primer paint/clear. The right way costs money and probibly should not be attemped if you are not sure on what you are doing. If its just surface rust the quick way is to clean up the rust I mean sand it down or sandblast it until its metal then some anti rust solution stuff that doesnt work, light putty (no bondo) or spot putty, and primer paint/clear. The rust probibly will come back but not for a while. Now if you have a hole and want to do it the cheap ass i will never tell anyone I did this way. Cut out the weak metel. Go buy some sheat metal that is thin enough to bend how you need. Cut it so it fits inside the rust hole rivit that piece of metal in place behind car metal, grind down the rivits so you can barley see them but so they still hold. Putty it up (no bondo) sand then prime paint clear. This should cover todays lesson of how to be a wanta be body man.
Good luck and if you have anyother questions we are all here to help!!!!
Da Soup