Radio buttons come in groups. They are bound together as a group by having the same name:

<INPUT TYPE="radio" NAME="rad" VALUE="radio_button1">foo
<INPUT TYPE="radio" NAME="rad" VALUE="radio_button2">bar
<INPUT TYPE="radio" NAME="rad" VALUE="radio_button3">zot
<INPUT TYPE="radio" NAME="rad" VALUE="radio_button4">harry

Gives:

foo
bar
zot
harry


A particular radio button can be preselected with the "CHECKED" attribute:

<INPUT TYPE="radio" NAME="rad" Value="rad_button1" CHECKED>

Or with Javascript:

form.rad[0].checked = true;  // sets to first button in the rad group

Finding the selected radio button with Javascript:

function test_radio_button (form){
    for (Count = 0; Count < 3; Count++) {
        if (form.rad[Count].checked)
            break;
    }
    alert ("Button " + Count + " is selected");
}