Beginner need help

hi everybody
firstable, i'm french so would you please forgive me for what i will try to explain !

i am beginning with python scrtipting for xbmc. something disturb me ... the variables (not sure of the word.. maybe memories is the correct one)
i don't know when and where i should declare one as global or not...
please if someone can explain a little bit, it would be very nice !
thanks a lot !