To put this folder on the PATH environment variable type
export PATH="$PATH:$HOME/.composer/vendor/bin"
This appends the folder to your existing PATH, however, it is only active for your current terminal session.
If you want it to be automatically set, it depends on the shell you are using. For bash, you can append this line to $HOME/.bashrc using your favorite editor or type the following on the shell
echo 'export PATH="$PATH:$HOME/.composer/vendor/bin"' >> ~/.bashrc
In order to check if it worked, logout and login again or execute
source ~/.bashrc
on the shell.
PS: For other systems where there is no ~/.bashrc, you can also put this into ~/.bash_profile
PSS: For more recent laravel you need to put $HOME/.config/composer/vendor/bin on the PATH.
PSSS: If you want to put this folder on the path also for other shells or on the GUI, you should append the said export command to ~/.profile (cf. https://help.ubuntu.com/community/EnvironmentVariables).