Plugin

                <script src="https://cdn.jsdelivr.net/jquery.validation/1.16.0/jquery.validate.min.js"></script>

1. Form

<form id="second_form" method="post" action="">
  <div>
    <label for="fname">First Name:</label>
    <input type="text" id="fname" name="fname"></input>
  </div>
  <div>
    <label for="lname">Last Name:</label>
    <input type="text" id="lname" name="lname"></input>
  </div>
  <div>
    <label for="user_email">Email:</label>
    <input type="email" id="user_email" name="user_email"></input>
  </div>
  <div>
    <label for="psword">Password:</label>
    <input type="password" id="psword" name="psword"></input>
  </div>
  <div>
    <input type="submit" value="Submit" />
  </div>
</form>

2. Script

1. Definde function

$('form[id="second_form"]').validate({

});

second_form is id of form

2. Write rules for mandatory fields

$('form[id="second_form"]').validate({

  rules: {
    fname: 'required',
    lname: 'required',
    user_email: {
      required: true,
      email: true,
    },
    psword: {
      required: true,
      minlength: 8,
    }
  }


});

We use the name of fields inside rules parameter

3. Write message for mandatory fields

$('form[id="second_form"]').validate({


  messages: {
    fname: 'This field is required',
    lname: 'This field is required',
    user_email: 'Enter a valid email',
    psword: {
      minlength: 'Password must be at least 8 characters long'
    }
  },


});  

We use the name of fields inside messages parameter

4. Write form submit

$('form[id="second_form"]').validate({

  submitHandler: function(form) {
    form.submit();
  }

});

Form submit() is used inside submitHandler parameter

3. Completed code

$('form[id="second_form"]').validate({
  rules: {
    fname: 'required',
    lname: 'required',
    user_email: {
      required: true,
      email: true,
    },
    psword: {
      required: true,
      minlength: 8,
    }
  },
  messages: {
    fname: 'This field is required',
    lname: 'This field is required',
    user_email: 'Enter a valid email',
    psword: {
      minlength: 'Password must be at least 8 characters long'
    }
  },
  submitHandler: function(form) {
    form.submit();
  }
});

4. Another example

$("#signupForm").validate({
               
               // In 'rules' user have to specify all the
              // constraints for respective fields
               rules: {
                   firstname: "required",
                   lastname: "required",
                   username: {
                       required: true,
                       minlength: 2 // For length of lastname
                   },
                   password: {
                       required: true,
                       minlength: 5
                   },
                   confirm_password: {
                       required: true,
                       minlength: 5,
                      
                       // For checking both passwords are same or not
                       equalTo: "#password"
                   },
                   email: {
                       required: true,
                       email: true
                   },
                   agree: "required"
               },
               // In 'messages' user have to specify message as per rules
               messages: {
                   firstname: " Please enter your firstname",
                   lastname: " Please enter your lastname",
                   username: {
                       required: " Please enter a username",
                       minlength:
                     " Your username must consist of at least 2 characters"
                   },
                   password: {
                       required: " Please enter a password",
                       minlength:
                     " Your password must be consist of at least 5 characters"
                   },
                   confirm_password: {
                       required: " Please enter a password",
                       minlength:
                     " Your password must be consist of at least 5 characters",
                       equalTo: " Please enter the same password as above"
                   },
                   agree: "Please accept our policy"
               }
           });
       });

      

      <form class="cmxform"
          id="signupForm"
          method="get"
          action="form-handler.html"
          autocomplete="off">
        <fieldset>
            <legend>GFG sign-up Form</legend>
 
            <p>
                <label for="firstname">
                      Firstname
                  </label>
                <input id="firstname"
                       name="firstname"
                       type="text">
            </p>
            <p>
                <label for="lastname">
                      Lastname
                  </label>
                <input id="lastname"
                       name="lastname"
                       type="text">
            </p>
            <p>
                <label for="username">
                      Username
                  </label>
                <input id="username"
                       name="username"
                       type="text">
            </p>
            <p>
                <label for="password">
                      Password
                  </label>
                <input id="password"
                       name="password"
                       type="password">
            </p>
            <p>
                <label for="confirm_password">
                  Confirm password
                </label>
                <input id="confirm_password"
                       name="confirm_password"
                       type="password">
            </p>
            <p>
                <label for="email">
                  Email
                  </label>
                <input id="email"
                       name="email"
                       type="email">
            </p>
            <p>
                <label for="agree">
                  Please agree to our policy
                  </label>
                <input id="agree"
                       name="agree"
                       type="checkbox">
            </p>
            <p>
                <input class="submit"
                       type="submit"
                       value="submit">
            </p>
 
        </fieldset>
    </form>