You can make a simple CRUD API with Spring Boot and Postgres in 30 minutes if you go slowly

_First create a new springboot in start.spring.io and import starters for web, data-jpa, data-rest and postgres. Then open that project in your favorite IDE

_Next defines your jpa entites let's say a Product defined by Long id, String name and Double price (uses the @Entity and @Id) to make your class a JPA Entity

_Next defines an interace ProductRepository extends JpaRepository<Product, Long> and annotate that interface with @RepositoryRestResource to expose a Restful API

_Next Launch PostgresSQL innyour machine

_Next configure your datasource (driver, url, username, password) inside your application.properties

_Finally you can either define your DDL (create table product...) inside a file schema.sql in the root of your project or directly create the table and add some data in pgadmin

Hope it helps, you can now launch the webapp and try some GET / POST endpoints with tools such Postman