in the store, the guy tell me that i may dont use the whammy bar because it will go out of tune or it can break the strings... i want you to tell me the risks of use the whammy bar, the solutions to go out of tune or string-break , the kind of string is the solution. ( sorry for mi bad english :S )
what kinda of guitar is it?
because whammy bars shouldn't break a string or go out of tune if its if its set up correctly

okay it might make the guitar go out of tune a bit but unless its a fender style or bigsby it shouldnt do too much
it one of the cheapests :S , its not the worst, but lets say its the WOOORST GUITAR IN THE WORLD, i need to change the strings for other? thanks to reply :P
Well, it depends on whether you have a floating bridge or not. A whammy bar used with a floating bridge should be okay. But please don't put too much pressure on the whammy bar and it should be okay.

What kind of guitar do you have??