String url = "http://54.37.137.53:8260/todolist/app/core/action/fixapi.php";
String charset = "UTF-8";  // Or in Java 7 and later, use the constant: java.nio.charset.StandardCharsets.UTF_8.name()
String method = "login";
String login = "admin@admin.pl";
String password = "admin123";
// ...

String query = String.format("method=%s&login=%s&password=%s",
     URLEncoder.encode(method, charset),
     URLEncoder.encode(login, charset),
     URLEncoder.encode(password, charset));

URLConnection connection = new URL(url).openConnection();
connection.setDoOutput(true); // Triggers POST.
connection.setRequestProperty("Accept-Charset", charset);
connection.setRequestProperty("Content-Type", "application/json;charset=" + charset);

try (OutputStream output = connection.getOutputStream()) {
    output.write(query.getBytes(charset));
}

InputStream response = connection.getInputStream();

System.out.print(response); // W ZMIENNEJ RESPONSE JEST WYNIK, SPRAWDZ CO W NIM SIE ZNAJDUJE