static TestMethod void DmlTest() {
    List<Contact> allContacts = [SELECT Id FROM Contact];
    System.assert(allContacts.isEmpty());

    Contact testContact = new Contact(FirstName = 'John', LastName = 'Doe');
    insert testContact;
    allContacts = [SELECT Id FROM Contact];
    System.assertNotEquals(0, allContacts.size(), 'Optional message in case of failure');
    
    delete allContacts;
    allContacts = [SELECT Id FROM Contact];
    System.assertEquals(0, allContacts.size());
}

This test class will test the IsBlank(...) method of SomeClass. Below is the example SomeClass. This class has only the one, basic static method, but you will be unable to deploy it to a production instance for use until you have reached the code coverage threshold.

public class SomeClass {

    public static Boolean IsBlank(String someData) {
        if (someData == null) {
            return true;
        } else if (someData == '') {
            return true; 
        } else {
            return false;
        }
    }

}

As one can see, this method is simply a if statement with three branches. To write an effective test class, we must cover each branch with code, and use System.assertEquals(...) statements to verify that the proper data was received from IsBlank(...).

@isTest 
public class SomeClass_test {

    @isTest 
    public static void SomeClass_IsBlank_test() {

        String testData;

        // SomeClass.IsBlank() returns true for Null values
        System.assertEquals(true, SomeClass.IsBlank(testData)); 

        testData = '';
        
        // SomeClass.IsBlank() returns true for empty strings
        System.assertEquals(true, SomeClass.IsBlank(testData)); 

        testData = 'someData';

        // SomeClass.IsBlank() returns false when testData is neither
        // an empty string nor Null
        System.assertEquals(false, SomeClass.IsBlank(testData)); 

    }

}

You can use a method annotated with @testSetup to write code that will have been executed before each test run:

public class AccountService {
  public static Account fetchAccount() {
    return [ SELECT Id, Name FROM Account LIMIT 1 ];
  }
}

@isTest
public class AccountServiceTest {
  private static final String TEST_ACCOUNT_NAME = 'My Test Account';

  @testSetup
  public static void setUpAccountData() {
    Account a = new Account(Name = TEST_ACCOUNT_NAME);
  }

  @isTest
  public static void testFetchAccount() {
    Account a = AccountService.fetchAccount();
    System.assertNotEquals(null, a, 'Account should not be null');
    System.assertEquals(TEST_ACCOUNT_NAME, a.Name, 'Account name should be correct');
  }
}

While you can use the @testSetup annotation to designate a method to be run before tests are executed, this method will usually only be run once. If you need code to be run before each test, you can use a static block:

@isTest
public class MyTest {
  static {
    // code here will be run before each test is executed
  }
}

System.assert can be used to check that a boolean expression evaluates to true:

System.assert(Service.isActive());
System.assert(!Service.getItems().isEmpty(), 'items should not be empty');

System.assertEquals and System.assertNotEquals can be used to check equality of two values. The expected value is passed as the first parameter, and the value under test is passed as the second.

System.assertEquals(4, Service.getItems().size());
System.assertNotEquals(null, Service.getItems());

// failure messages are optional:
System.assertEquals(true, Service.doWork(), 'doWork should return true');
System.assertNotEquals(null, Service.doWork(), 'doWork should not be null');