First alternative is to use GUIDE. This will allow you to set a specific radio button to "selected" everytime the GUI initializes. To do this, open the "Property Inspector" for the radio button and change its "Value" to 1.
Second alternative is to do it programatically. In your code, wherever you initialize the button-group, execute the following line:
Where, "radioTag" is whatever tag you assigned to your desired radio button when making the gui.