Although earlier versions of Struts have a utility for setting up a DataSource in the struts-config.xml file, I'd strongly advise you not to use it. It's deprecated in later versions, and it just plain doesn't work very well.
The best practice for using a database in any J2EE application is to set up a DataSource in the Application Server's Java Naming and Directory Interface (JNDI), and then use JNDI in your application to "look up" the DataSource and use it. The process for setting up a DataSource in JNDI is a little different for each application server. Consult the documentation for the App server you're using for instructions on how to do it.