HTML/PHP contact form will not submit

When I hit send on the form on the live website it refreshes and clears the input fields, but I never get the form in my email. I am new to using PHP so I have basically been trying code that other users have provided. This code so far is the only one that will refresh the page, but still does not send to my email.

Here is the html code:

<div class="col-md-4">
                <div class="contact_right">
            <div class="contact-form_grid">
               <form method="post" action="">
                 <input type="text" class="textbox" value="Your name" onfocus="this.value = '';" onblur="if (this.value == '') {this.value = 'Your name';}">
                 <input type="text" class="textbox" value="Your email" onfocus="this.value = '';" onblur="if (this.value == '') {this.value = 'Your email';}">
                 <input type="text" class="textbox" value="Subject" onfocus="this.value = '';" onblur="if (this.value == '') {this.value = 'Subject';}">
                 <textarea value="Message:" onfocus="this.value = '';" onblur="if (this.value == '') {this.value = 'Message';}">Message</textarea>
                 <input type="submit" value="Send">
               </form>

AND I WOULD LOVE TO BE ABLE TO SHOW SOMEONE THE PHP CODE BUT I CANNOT FIGURE OUT HOW TO ADD IT INTO MY POST???

Any help would be greatly appreciated.