Is there a secret to become a good Tango dancer ?

No there is no secret. Just work, take lessons, work, go to the ball, work, establish a good relation with your partner, and work ever and ever. Be also conscious that if your Tango professor only teaches you sequences after sequences, run away as fast as you can, because you won't learn to dance with sequences. If your target is to be able to dance in a ball, forget the sequences. If your target is to dance on stage, then maybe some sequences may help you but not that much. Always keep in mind that learning via sequences will limit you to copy.

What are the main bases in the process of learning Tango ?

First, you need to find a good teacher and second be patient ! Be patient with yourself and be patient with your partner whatever your and her/his level.