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

"composer -V"

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

Drush installation
Change your directory to directory /usr/local using following command

"cd /usr/local"

Clone git repository for drush

"sudo git clone https://github.com/drush-ops/drush.git"

Move to Drush root directory.

"cd drush"

Change permission for drush

"sudo chmod +x drush"

Install composer for drush

"sudo composer install"

Hope this helps !!!