Getting sidebar in new custom theme to sit on side instead of bottom

Hello

I’m having a problem getting my sidebar in a theme I’m developing from scratch to sit on the side. I’m completely new to WordPress so I’m trying to teach myself how to navigate by doing a custom theme.

Here is the index page code:

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#header {font-size:15px;
font-family: mono;
color:yellow;
text-align: center;
}

#sidebar {
list-style: none;
font-family: 'Times New Roman';
color: blue;
float: right;
}

#tagline {
font-family: courier;
text-align: center ;
}

#content {
font-family: arial;
color: pink;
text-decoration:none;
}

#nav {
font-family: courier;
color: red;
list-style: none;
text-align: center
}

#footer {
font-family: courier;
text-align: center;
}

And the sidebar.php file:

<ul id="sidebar">
 <li id="about">
  <h2>About</h2>
  <p>This is my blog.</p>

 <li id="links">
  <h2>Links</h2>

<ul>
<li><a href="https://example.com">Example</a></li>
</ul>