Drush depends in various packages provided by composer. First we need to check if composer is already installed. You can find it out by typing the following command
If it gives you the version number then its already installed. If not then follow the following steps to get composer installed in your system
curl -sS https://getcomposer.org/installer | php
sudo mv composer.phar /usr/local/bin/composer
Change your directory to directory /usr/local using following command
Clone git repository for drush
"sudo git clone https://github.com/drush-ops/drush.git"
Move to Drush root directory.
Change permission for drush
"sudo chmod +x drush"
Install composer for drush
"sudo composer install"
Hope this helps !!!