Шаг 1) добавить HTML:
Используйте элемент <form> для обработки входных данных.

 <div class="row">

   <div class="col-75">

     <div class="container">

       <form action="/action_page.php">

 

         <div class="row">

           <div class="col-50">

             <h3>Billing Address</h3>

             <label for="fname"><i class="fa fa-user"></i> Full Name</label>

             <input type="text" id="fname" name="firstname" placeholder="John M. Doe">

             <label for="email"><i class="fa fa-envelope"></i> Email</label>

             <input type="text" id="email" name="email" placeholder="john@example.com">

             <label for="adr"><i class="fa fa-address-card-o"></i> Address</label>

             <input type="text" id="adr" name="address" placeholder="542 W. 15th Street">

             <label for="city"><i class="fa fa-institution"></i> City</label>

             <input type="text" id="city" name="city" placeholder="New York">

 

             <div class="row">

               <div class="col-50">

                 <label for="state">State</label>

                 <input type="text" id="state" name="state" placeholder="NY">

               </div>

               <div class="col-50">

                 <label for="zip">Zip</label>

                 <input type="text" id="zip" name="zip" placeholder="10001">

               </div>

             </div>

           </div>

 

           <div class="col-50">

             <h3>Payment</h3>

             <label for="fname">Accepted Cards</label>

             <div class="icon-container">

               <i class="fa fa-cc-visa" style="color:navy;"></i>

               <i class="fa fa-cc-amex" style="color:blue;"></i>

               <i class="fa fa-cc-mastercard" style="color:red;"></i>

               <i class="fa fa-cc-discover" style="color:orange;"></i>

             </div>

             <label for="cname">Name on Card</label>

             <input type="text" id="cname" name="cardname" placeholder="John More Doe">

             <label for="ccnum">Credit card number</label>

             <input type="text" id="ccnum" name="cardnumber" placeholder="1111-2222-3333-4444">

             <label for="expmonth">Exp Month</label>

             <input type="text" id="expmonth" name="expmonth" placeholder="September">

 

             <div class="row">

               <div class="col-50">

                 <label for="expyear">Exp Year</label>

                 <input type="text" id="expyear" name="expyear" placeholder="2018">

               </div>

               <div class="col-50">

                 <label for="cvv">CVV</label>

                 <input type="text" id="cvv" name="cvv" placeholder="352">

               </div>

             </div>

           </div>

 

         </div>

         <label>

           <input type="checkbox" checked="checked" name="sameadr"> Shipping address same as billing

         </label>

         <input type="submit" value="Continue to checkout" class="btn">

       </form>

     </div>

   </div>

 

   <div class="col-25">

     <div class="container">

       <h4>Cart

         <span class="price" style="color:black">

           <i class="fa fa-shopping-cart"></i>

           <b>4</b>

         </span>

       </h4>

       <p><a href="#">Product 1</a> <span class="price">$15</span></p>

       <p><a href="#">Product 2</a> <span class="price">$5</span></p>

       <p><a href="#">Product 3</a> <span class="price">$8</span></p>

       <p><a href="#">Product 4</a> <span class="price">$2</span></p>

       <hr>

       <p>Total <span class="price" style="color:black"><b>$30</b></span></p>

     </div>

   </div>

 </div>

Шаг 2) добавить CSS:
используйте CSS Flexbox для создания адаптивного макета:

 .row {

   display: -ms-flexbox; /* IE10 */

   display: flex;

   -ms-flex-wrap: wrap; /* IE10 */

   flex-wrap: wrap;

   margin: 0 -16px;

 }

 

 .col-25 {

   -ms-flex: 25%; /* IE10 */

   flex: 25%;

 }

 

 .col-50 {

   -ms-flex: 50%; /* IE10 */

   flex: 50%;

 }

 

 .col-75 {

   -ms-flex: 75%; /* IE10 */

   flex: 75%;

 }

 

 .col-25,

 .col-50,

 .col-75 {

   padding: 0 16px;

 }

 

 .container {

   background-color: #f2f2f2;

   padding: 5px 20px 15px 20px;

   border: 1px solid lightgrey;

   border-radius: 3px;

 }

 

 input[type=text] {

   width: 100%;

   margin-bottom: 20px;

   padding: 12px;

   border: 1px solid #ccc;

   border-radius: 3px;

 }

 

 label {

   margin-bottom: 10px;

   display: block;

 }

 

 .icon-container {

   margin-bottom: 20px;

   padding: 7px 0;

   font-size: 24px;

 }

 

 .btn {

   background-color: #4CAF50;

   color: white;

   padding: 12px;

   margin: 10px 0;

   border: none;

   width: 100%;

   border-radius: 3px;

   cursor: pointer;

   font-size: 17px;

 }

 

 .btn:hover {

   background-color: #45a049;

 }

 

 span.price {

   float: right;

   color: grey;

 }

 

 /* Responsive layout - when the screen is less than 800px wide, make the two columns stack on top of each other instead of next to each other (and change the direction - make the "cart" column go on top) */

 @media (max-width: 800px) {

   .row {

     flex-direction: column-reverse;

   }

   .col-25 {

     margin-bottom: 20px;

   }

 }