Derailleur adjustments is one of those things where it is easier to show someone in person than it is to explain with types words, so see if this will help:
http://www.parktool.com/blog/repair-...nts-derailleur

Sometimes when I have or get a bike that is just not cooperating then I just have to take it back to the starting point and disconnect the cable from the derailleur, re-set any adjustment/tension barrels to nominal, verify that the shifter mechanism is working properly by using hand tension on the cable and working the shifter to verify that I can feel the cable move in or out with each click of the shifter, then setting the derailleur limit screws by pushing the derailleur by hand (watch out for your fingers around the cogs and chain) and ensuring the derailleur lines up with the smallest and largest cogs at eacg end of the range limited by the limit screws, then re-attaching the cable to the derailleur with a little tension on the cable, then fine tuning the shifting with any barrel cable housing adjusters if your bike has those.

Like I said, easier to show in person than with typed words. Hopefully the ParkTool link will help.

A long shot, since your location says you're in Paris, would be to go to the finish of the Tour de France tomorrow, find one of the team mechanics and ask them for a lesson since the race will be over then. And if you do manage this then post a pic of your Birdie on a team repair stand.

There could be other issues such as a worn our derailleur, worn pivot pins, bent hanger, worn shifter, stretchy cable housing, frayed and stretching cable, and so forth. Give the ParkTool link a read and see it that helps.