Change appearance of search form
To design the search block form you can override the existing search-block-form.tpl.php in the theme's template folder. 

&lt;?php

$variables['form']['search_block_form']['#attributes']['placeholder'] = "Search for...";
$variables['form']['search_block_form']['#attributes']['class'] = "form-control";

?&gt;

&lt;div class="row"&gt;
  &lt;div class="col-md-3"&gt;
    &lt;div class="input-group"&gt;      
      &lt;span class="input-group-btn"&gt;
        &lt;?php if (empty($variables['form']['#block']-&gt;subject)): ?&gt;
          &lt;button class="btn btn-default" type="button"&gt;&lt;?php print t('Go!');?&gt;&lt;/button&gt;
        &lt;?php endif; ?&gt;
        &lt;?php print $search_form; ?&gt;
      &lt;/span&gt;
    &lt;/div&gt;
  &lt;/div&gt;
&lt;/div&gt;