Custom CMS how to create categories for posts with PHP/MySQL database

I am searching all around the internet but I can't seem to find a simple tutorial that can help me with my custom CMS I am building.

I'd like to add multiple categories to my posts.

For example:

Title: post 1 
Content: content goes here 
Categories: Technology, Computers, Science, Internet

Title: post 2 
Content: content goes here 
Categories: Music, Jazz, Classic

So this is what I have to post my articles

My database structure

postID | postTitle | postDesc | postCont | postDate


1   Post 1      info1   content1    date1
2   Post 2      info2   content2    date2
3   Post 3      info2   content3    date3

The FORM

<form action='' method='post'>

<p><label>Title</label><br />
<input type='text' name='postTitle' value='<?php if(isset($error)){ echo $_POST['postTitle'];}?>'></p>

<p><label>Description</label><br />
<textarea name='postDesc' cols='60' rows='10'><?php if(isset($error)){ echo $_POST['postDesc'];}?></textarea></p>

<p><label>Content</label><br />
<textarea name='postCont' cols='60' rows='10'><?php if(isset($error)){ echo $_POST['postCont'];}?></textarea></p>

<p><input type='submit' name='submit' value='Submit'></p>

</form>

the PHP code to place it in the database

//insert into database
$stmt = $db->prepare('INSERT INTO blog_posts (postTitle,postDesc,postCont,postDate) VALUES (:postImage, :postTitle, :postDesc, :postCont, :postDate)') ;
$stmt->execute(array(
':postTitle' => $postTitle,
':postDesc' => $postDesc,
':postCont' => $postCont,
':postDate' => date('Y-m-d H:i:s')
));

//redirect to index page
header('Location: index.php?action=added');
exit;