Since you're asking about server.xml I presume your application server is Tomcat. If you look on the Tomcat website you will find copious documentation explaining step-by-step how to achieve this for each likely Tomcat and Database combination.
If you can't find the Tomcat website, use Google.
If you can't find Google, it's here: http://google.com/
If you have further problems, having consulted the documentation, come back here and ask a specific question.