To create a datasource and a connection pool to help you use databases in your project, edit the $TOMEE_HOME/conf/tomee.xml file.

Inside the <tomee> node, insert a child node like this one:

  <Resource id="MyDataSouceDs" type="javax.sql.DataSource">
      jdbcDriver = org.postgresql.Driver
      jdbcUrl = jdbc:postgresql://[host]:[port]/[database]
      jtaManaged = true
      maxActive = 20
      maxIdle = 20
      minIdle = 0
      userName = user
      password = password
  </Resource>

The example targets PostgreSQL, so you will need to perform some changes for another database. Of course, you will also need to change the other parameters according to your needs.