php - How to hash a password field

bought a php software that allows users to build websites. But the guy I paid didn't build a signup form. I'm dangerous enough to do basic coding. But I feel this is past my skill level. The form I created inserts into the mysql database just fine, but it wold let the new users that use this form login because the password field isn't hashing.

HMTL signup form:

            
Fillout the below to join.

            

                First Name
                
                
            

            

                Last Name

                
                
            

            

                Email
                
                
            


            

                Password
                
                
            

            

                
                
            

        

and the php script:

$con = mysql_connect("localhost","name","password");
if (!$con)
  {
  die('Could not connect: ' . mysql_error());

  }

mysql_select_db("instantw_builder", $con);


$sql="INSERT INTO users (first_name, last_name, username, email, password)
VALUES
('$_POST[first_name]','$_POST[last_name]','$_POST[email]','$_POST[email]','$_POST[password]')";

if (!mysql_query($sql,$con))

  {
  die('Error: ' . mysql_error());
  }
echo "1 record added";

mysql_close($con)
?>

What do I need to do to hash the password field?