Insted of directly giving an answer, I provides some pointers,
Simplest one, Google it.
Secondly, download the DTD for your existing web.xml, search through that DTD regarding where to define tlds.
(Hint: it ll look kike :http:/java.sun.com/j2ee/dtds/web-app_2_X.dtd, where x is there version you're using)
Finally, You need NOT have to define a TLDs uri entry in web.xml, if you're using JSP2.0 and above complaint container/server.