To create a datasource and a connection pool to help you use databases in your project, edit the $GLASSFISH_HOME/glassfish/domains/[domain_name]/config/domain.xml file. Inside the <resources> node, insert a child node like this one:

<jdbc-connection-pool 
  pool-resize-quantity="4" 
  max-pool-size="64" 
  max-wait-time-in-millis="120000" 
  driver-classname="com.mysql.jdbc.Driver" 
  datasource-classname="com.mysql.jdbc.jdbc2.optional
  .MysqlDataSource" 
  steady-pool-size="16" 
  name="MysqlPool" 
  idle-timeout-in-seconds="600" 
  res-type="javax.sql.DataSource">
      <property name="databaseName" value="database"></property>
      <property name="serverName" value="[host]"></property>
      <property name="user" value="user"></property>
      <property name="password" value="password"></property>
      <property name="portNumber" value="3306"></property>
</jdbc-connection-pool>
<jdbc-resource pool-name="MySqlDs" jndi-name="jdbc/MySqlDs">
</jdbc-resource>

Then, look for this node:

<server config-ref="server-config" name="server">

Add this child to it:

<resource-ref ref="jdbc/MySqlDs"></resource-ref>

The example targets MySQL, 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.