In order to use fontawesome icons, you need to add
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css" integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/" crossorigin="anonymous"> into your <head> section of the HTML.
Then, you can use any icon, for example add
<i class="fas fa-adjust"></i> in the <body> where you need it.