Oi Lucas
Para inicializar a JPA, é preciso definir um arquivo de configuração, chamado persistence.xml. Nele você precisa criar o persistence-unit "contas" conforme abaixo:
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="contas">
<class>br.com.alura.jpa.modelo.Conta</class>
<properties>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost/alura_jpa?serverTimezone=UTC" />
<property name="javax.persistence.jdbc.user" value="root" />
<property name="javax.persistence.jdbc.password" value="" />
<property name="hibernate.hbm2ddl.auto" value="update"/>
<property name="hibernate.dialect" value="org.hibernate.dialect.MariaDBDialect"/>
<property name="hibernate.show_sql" value="false"/>
<property name="hibernate.format_sql" value="true"/>
</properties>
</persistence-unit>
</persistence>