Run this in your terminal to get the latest Composer version:
curl -sS https://getcomposer.org/installer | php
Or if you don't have curl:
php -r "readfile('https://getcomposer.org/installer');" | php
This installer script will simply check some php.ini settings, warn you if they are set incorrectly, and then download the latest composer.phar in the current directory.
For composer documentation, please refer to getcomposer.org