You should go through the Getting started with Bootstrap and Quick Start Guide
As per the getting started guide,
Bootstrap-select requires jQuery v1.9.1+, Bootstrap’s dropdown.js component, and Bootstrap's CSS. If you're not already using Bootstrap in your project, a precompiled version of the Bootstrap v3.3.7 minimum requirements can be downloaded here. If using bootstrap-select with Bootstrap v4+, you'll also need Popper.js. For all of Bootstrap v4's requirements, see Getting started. A precompiled version of the requirements will be made available in an upcoming release of bootstrap-select.
https://codepen.io/yudizkkw/pen/GRKYdjz?editors=1010
<html>
<head>
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap-select@1.13.9/dist/css/bootstrap-select.min.css">
<!-- Latest compiled and minified JavaScript -->
<script src="https://cdn.jsdelivr.net/npm/bootstrap-select@1.13.9/dist/js/bootstrap-select.min.js"></script>
<!-- (Optional) Latest compiled and minified JavaScript translation files -->
</head>
<body>
<select class="selectpicker">
<option>Mustard</option>
<option>Ketchup</option>
<option>Relish</option>
</select>
</body>
</html>