Deformat,
Everyone here has offered good advice on where to start. I too, am against the idea of working on a smartphone/tablet as a beginning designer. For the simple reason that you won’t be able to make a prototype and see and test the results. Design is not just making pretty pictures. It is about finding solutions to peoples “problems”. The result is a physical object.
After you have considered and designed a solution, you want to know how good your idea is. You want to test it and feel it and watch other people test it and use it. With a smartphone, or a tablet, or a piece of very complicated technology, you are not able to see the result unless you work in a lab at a very big technology company. To grow as a designer you want to make you own work better through evolution.
As a young person intent on learning design, try to pick something that you could work all the way through, even if it is a rough prototype. Konstantin Grcic builds great models out of cardboard and glue as prototypes. Technology should not be a limitation.
Best of luck to you, great to be able to reach all these people all over the world for input.