just download composer.phar and execute

composer.phar create-project laravel/laravel your-project-name --prefer-dist

But my old installation of composer in usr/local/bin? i just delete it with sudo rm composer?

No.. this means you have composer installed globally (i.e. simply typing in composer in any terminal window should display the composer menu options. refer to these links if you have composer installation issues: https://getcomposer.org/doc/00-intro.md#globally and http://askubuntu.com/a/165241)

So, open up a terminal window.. Navigate to /var/www (default webroot for apache in ubuntu) and then type in:

composer create-project laravel/laravel your-project-name --prefer-dist

Thanks!

I did it but I obtain this error(if i didn't use sudo permission)

[ErrorException]
unlink(/home/pietro/.composer/cache/files/laravel/laravel/73094f2633f1b90f3
ef6de4a8a5b610532510e0e.zip): Permission denied