In Tomcat find conf/context.xml, then edit and write something like this:
<Resource name="jdbc/PollDatasource" auth="Container" type="javax.sql.DataSource"
driverClassName="com.YourDriver"
url="jdbc:derby://localhost:1527/nameOfTheDatabase;create=true"
username="username" password="password" maxActive="20"
maxIdle="10" maxWait="-1" />
Note 1: With the above URL the driver will be org.apache.derby.jdbc.ClientDriver
Note 2 : You can also add the above information in META-INF/context.xml of your project. This becomes application specific.If you add the information in tomcat's context.xml that becomes global.
Note 3: Download the jar from this website.Download db-derby-10.9.1.0-bin.zip.It contains many files, including derby.jar and derbyclient.jar (along with much documentation).derbyclient.jar contains our friend org.apache.derby.jdbc.ClientDriver.class. derby.jar contains org.apache.derby.jdbc.EmbeddedDriver. Keep the downloaded jar in lib folder of Tomcat.
and in your application web.xml "resource-ref":
<resource-ref>
<description>my connection</description>
<res-ref-name>jdbc/PollDatasource</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
You may want to look at these questions :