Comparisons between two strings don't work
For starters this call
fflush(stdin);
has undefined behavior.
The function fgets can append the new line character '\n' to the entered string. You need to remove it. For example
choix_utilisateur[ strcspn( choix_utilisateur, "\n" ) ] = '\0';
Pay attention to that the conditions in if statements will be more readable if for example instead of this condition
if(!strcmp(choix_utilisateur, "toto")){
to write
if( strcmp(choix_utilisateur, "toto") == 0 ){