Primeiro configure o maven. Este é minha configuração completa:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>com.example</groupId>
	<artifactId>demo-spring-security</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>jar</packaging>

	<name>demo-spring-security</name>
	<description>Demo project for Spring Boot</description>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.0.6.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>

	<properties>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
		<java.version>1.8</java.version>
	</properties>

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-freemarker</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-security</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

</project>

A classe principal ficou da seguinte forma:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;

@SpringBootApplication
@Controller
public class DemoSpringSecurityApplication {

	public static void main(String[] args) {
		SpringApplication.run(DemoSpringSecurityApplication.class, args);
	}

	@GetMapping("/")
	public String teste() {
		return "index";
	}

	@GetMapping("/home")
	public String home() {
		return "home";
	}

	@GetMapping("/login")
	public String login() {
		return "login";
	}
}

Note que utilizei a classe principal também como controller.

A configuração do Spring Security do projeto:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfiguration;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
import org.springframework.security.core.userdetails.MapReactiveUserDetailsService;
import org.springframework.security.core.userdetails.ReactiveUserDetailsService;
import org.springframework.security.core.userdetails.User;

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity httpSecurity) throws Exception {
        httpSecurity
                .authorizeRequests().antMatchers("/").permitAll()
                .anyRequest().authenticated()
                .and()
                .formLogin().loginPage("/login").permitAll()
                .and()
                .logout().logoutUrl("/logout").permitAll();

        httpSecurity.csrf().disable();
        httpSecurity.headers().frameOptions().disable();
    }

    @Autowired
    public void configureGlobal(AuthenticationManagerBuilder auth) throws Exception {
        auth
                .inMemoryAuthentication()
                .withUser("admin").password("{noop}admin").roles("ADMIN")
                .and().withUser("user").password("{noop}user").roles("USER");
    }
}

Note que utilizamos {noop} antes da senha para indicar que não será criptografada.

E por fim as telas em HTML que devem ser criadas dentro da pasta template.

home.ftl

<h1>Logou no sistema</h1>

index.ftl

<h1>Tela inicial</h1>

login.ftl

<h1>Login</h1>

<form action="/login" method="post">

	<label> User Name : <input type="text" name="username" value="admin"/> </label>
	<label> Password : <input type="password" name="password" value="admin"/> </label>
	<button type="submit" class="btn btn-default">Sign In</button>

</form>

Com isso já temos a aplicação funcionando com um exemplo de autenticação em memória.

Não esqueca de deixar suas dúvidas nos comentários e compartilhar este post.