try this example :
test.jsp
<form name="f">
<select name="testselect" onChange="window.location='test.jsp?val='+document.f.testselect.value">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
</form>
<%
String val= "";
if(request.getParameter("testselect") != null)
val = request.getParameter("testselect");
%>
// check for null before assigning values..good practice..

)
<form name="f2">
<input type="text" value="<%=val%>">
//the value of val will be printed in the text box of the second form
</form>
i hope this will solve ur problem