Attribute for <INPUT ...>
CHECKED

this code	produces this
<FORM ACTION="../cgi-bin/mycgi.pl"> <INPUT TYPE=CHECKBOX NAME="maillist" CHECKED>Yes! Put me on the list! <P><INPUT TYPE=SUBMIT VALUE="submit"> </FORM>	Yes! Put me on the list!
<FORM ACTION="../cgi-bin/mycgi.pl"> What color would you like?<BR> <INPUT TYPE=RADIO NAME="color" VALUE="green" >Green<BR> <INPUT TYPE=RADIO NAME="color" VALUE="red" >Red<BR> <INPUT TYPE=RADIO NAME="color" VALUE="blue" CHECKED >Blue<BR> <INPUT TYPE=RADIO NAME="color" VALUE="brown" >Brown <P><INPUT TYPE=SUBMIT VALUE="submit"> </FORM>	What color would you like? Green Red Blue Brown

Checkboxes are off by default. There is no default for which radio button is on when the form loads; most browsers have none on, some put the first one on.