Need help with databases in plugin development.

Hello,
I’m developing rating plugin for school project and I can’t figure it out how to properly connect and store data in database.

May you help me with providing some good tutorials suitable for beginners?

Right now I’m trying this way, which doesn’t work and as far as I know it’s wrong for WordPress.

?php
$ipaddress = md5($_SERVER['REMOTE_ADDR']); // here I am taking IP as UniqueID but you can have user_id from Database or SESSION

$servername = "localhost"; // Server details
$username = "root";
$password = "";
$dbname = "events";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("Unable to connect Server: " . $conn->connect_error);
}

if (isset($_POST['rate']) && !empty($_POST['rate'])) {

    $rate = $conn->real_escape_string($_POST['rate']);
// check if user has already rated
    $sql = "SELECT <code>id</code> FROM <code>tbl_rating</code> WHERE <code>user_id</code>='" . $ipaddress . "'";
    $result = $conn->query($sql);
    $row = $result->fetch_assoc();
    if ($result->num_rows > 0) {
        echo $row['id'];
    } else {

        $sql = "INSERT INTO <code>tbl_rating</code> ( <code>rate</code>, <code>user_id</code>) VALUES ('" . $rate . "', '" . $ipaddress . "'); ";
        if (mysqli_query($conn, $sql)) {
            echo "0";
        }
    }
}
$conn->close();
?>

Thank you!