I found this from a site called 9lessons.info
https://www.9lessons.info/2016/04/php-lo...ction.html I skimmed over it and this looks like it can at least get you started in the right direction. You can find a lot of examples you can learn from online if you look up PHP session login. If you do implement anything, make sure you are doing it with PDO and not mysql prepares and remember to never trust anything the user submits so you don't open yourself up to XSS.
Some personal tips from when I have implemented systems like this in the past, don't store plain text passwords in your database table use a hash of some sort and salt it too. On login you can store a user id in the PHP session array that you can use to identify the user with. Once you get a user system in place a good idea would be to create a class that represents a user and pulls data from the user table. This will help you when creating your admin system for example.
$sessUser = $db>selectUser($_SESSION['username']);
$activeUser = new User($sessUser);
db being a controller class that handles pulling data from your database and User being a class that represents a user. Then you can for example have a method in the user class that checks user privileges like:
//if user privallage is 2 then do some action
if($activeUser->getPrivileges() == 2){
//Do something here
}else{ //not admin maybe header to index?
header('Location: index.php');
}