In your component html file, put the following code at the top of your function:
JHTML::_('behavior.calendar');
Then for your date field in your form:
input class="inputbox" type="text" name="startdate" id="startdate" size="25" maxlength="25" value="" &
input type="reset" class="button" value="..." onclick="return showCalendar('startdate','%Y-%m-%d');"