If we assume that you have a REST robot and want to set the result format then there is a couple of ways of doing this. One is to set the default result format (or output format), this is done in RoboMaker (File->Configure Robot->Output formats).

No matter what the default result format is you can set the result format you want once you execute the robot. If no result format is specifically choosen your robot automatically use the default result format. The easiest way to see what possible result formats you have for your robot is to take "Try it" from the robots page in the Robot Gallery and then go to the tab "Result Format". There you see a dropdown of the possible result formats. You can use all these result formats when executing your robot by specifying the "resultformat" parameter to the robot URL, for example http://service.openkapow.com/allaf/getchiefsofstate?country=Kazakhstan&resultformat=xml or http://service.openkapow.com/allaf/getchiefsofstate?country=Kazakhstan&resultformat=csv (same robot, different result formats).

I hope this helped!