How to make simple login system with php

<form action="register.php" method="POST" autocomplete="off" id="frmContactus" class="contact__form mt-8 mt-lg-10 text-start">
                            <div class="d-flex flex-column gap-5 gap-lg-6">
                                <div class="row g-5 g-lg-6">
                                    <div class="col-sm-6 col-md-12 col-xl-6">
                                        <div class="single-input">
                                            <label class="mb-2 nw1-color" for="fname">Ime</label>
                                            <input type="text" class="fs-six-up bg_transparent" name="fname" id="fname" placeholder="Name" required>
                                        </div>
                                    </div>
                        </form>

<?php
// Database credentials
$servername = "";
$database = ""; // 
$username = ""; // 
$password = ""; //


$conn = mysqli_connect($servername, $username, $password, $database);


if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $fname = mysqli_real_escape_string($conn, $_POST['fname']);
    $lname = mysqli_real_escape_string($conn, $_POST['lname']);
    $uname = mysqli_real_escape_string($conn, $_POST['uname']);
    $password = mysqli_real_escape_string($conn, $_POST['password']);
    $cpassword = mysqli_real_escape_string($conn, $_POST['cpassword']);
    $email = mysqli_real_escape_string($conn, $_POST['email']);