<header>Üst alan</header>
<div class="kapsayici">
  <main class="iceri-alani">
    There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable.
  </main>
  <nav class="menu">Sol Menü</nav>
  <aside class="reklam">Sağ alan</aside>
</div>
<footer>Alt Alan</footer>

body {
  margin: 0;
  display: flex;
  min-height: 100vh;
  flex-direction: column;
}

header {
  background-color:#CCCCCC;
}

header,
footer {
  flex: none;
  padding: 1em;
}

.kapsayici {
  display: flex;
  flex: 1 0 auto;
  flex-direction: column;
}

.iceri-alani {
  background-color: #f78200;
}

.menu,
.reklam {
  padding: 1em;
  background: #39b648;
}

footer {
  background-color:#cfc;
}

.menu {
  order: -1;
  background-color: #449fdc;
}

@media (min-width: 768px) {
  .kapsayici {
    flex-direction: row;
  }
  .iceri-alani {
    flex: 1;
    padding: 20px;
    margin: 0;
  }
  .menu, .reklam {
    flex: 0 0 12em;
  }
}