To retrieve form parameter in a JSP page use request.getParameter("param") where param is the name of the parameter.
To automatically submit a page when a item is selected in a drop-down use the onChange event of DHTML. Then use javascript to submit the form.
Example:
<form name="testForm" action="action.jsp">
<select name="testSelect" onChange="javascript:document.forms['testForm'].submit();">
<option value="1">One</option>
<option value="2">Two</option>
</select>
</form>
I didn't
test it so if it doesn't work exactly... oh well.