This code fixes the header and footer.
CSS:
footer {
 width: 100%;
 position: fixed;
 bottom: 0;
 right: 0;
 background: black;
 color: white;
 height: 40px;
 display: flex;
}
header {
 width: 100%;
 position: fixed;
 top: 0;
 left: 0;
 color: white;
 background: black;
 height: 40px;
}
HTML:
<html>
 <body>
  <header>
   Este é o cabeçalho
  </header>
   este é o corpo
  <footer>
    Este é o rodapé
  </footer>
 </body>
</html>
The important CSS rules for the desired purpose are:
position: fixed
top: 0
bottom: 0
right: 0
left: 0
They instruct the code to fix the elements and in which positions.
From this, it is easy to adapt to your case.