<div class="pizza_name_div">
    <input type="text" name="pizza_name" id="pizza_name" placeholder="Enter your pizza name as u like. i.e : my-pizza" value="">
    <input type="submit" value="Go" id="pizza_name_submit" onclick="pizza_name()">
</div>

function pizza_name() {
  if( pizza_name != "" ) {
     .........
  }else{
    alert( "please enter a name" );
  }
}