After i made a help's program on Petit computer which describe all the command of Petit Computer, it helps me to know all the possibility i have. (can't give you QR codes because it's all in french). Very useful when you have only your console and can't consult the manual on your PC. Create a similar program and you learn a lot, just by doing it. (Function Guide)
Then i consult some tutorials on the web or some Petit computer's games for specifics things. But most of the time, code is just solve problems by transforming a sentence in code. So you need to understand IF THEN ELSE, Label, GOTO and GOSUB in priority.
And if you don't understand a command, make some tests to see how it works.
It depends, if you buy "SmileBasic" (Petit computer 3) on your 3DS, then yes learn it.
If you hesitate to buy it and you have "Petit computer" on your console 3DS or DSi, try to do something with it, the language is easy to understand compare with languages on PC. (and it'll prepare you for "Smile Basic" if you want to buy it)
If you want to do games on your PC, i can't recommend you any language to learn, i code only on to my console DSi for the moment.