Where I can find the previous Quantum-Experiment tutorial for beginners? (the one that was under https://quantumexperience.ng.bluemix.net/qstage/#/account)
All the tutorials are now on the IBM Q Experience website. They are in the form of Jupyter Notebooks to help you learn about programming with Qiskit. There are also some additional materials if you click the
? button, such as links to documentation about how to get started using the composer.