we can download magento2 in 3 ways.
1) Direct Download
2) Using COMPOSER
3) Using GIT

Step1: First you need to check composer install on system or not.
Type below command on your console to check composer installation

	composer -help

If not install than install it . using below command.

curl -sS https://getcomposer.org/installer | php
mv composer.phar /usr/local/bin/composer

The first command install Composer into your local directory,
the second will move the composer.phar file to a directory
where it can be accessed globally by your system.

Again check below command

 composer -help

Step 2: Install magento project on local machine, Type command on console.

composer create-project magento/project-community-edition magentodemo

  
root@system:/var/www/html# composer composer create-project magento/project-community-edition magentodemo

you see below output on your console

Installing magento/project-community-edition (2.0.2)
  - Installing magento/project-community-edition (2.0.2)
    Downloading: 100%         

Created project in Magento
Loading composer repositories with package information
    Authentication required (repo.magento.com):       
      Username: bfba1bb4bdfdcb63d8458457728f
      Password: 
Do you want to store credentials for repo.magento.com in /home/gaurav/.composer/auth.json ? [Yn] Y

Step3: After this you need username and password

 

https://www.magentocommerce.com/

and login into your account and than go to Develoers >> Secure Keys and generate key here.
Basically public key is belong to Username
Private key is belong to Password

Step4: Give the permission of the folder

sudo chmod -R 777 magentodemo/

Step5: After that put below url on your local browser.

http://localhost/magentodemo/

If you want to add sample data on your project than run
below command on console after complete install project.

php <your Magento install dir>/bin/magento sampledata:deploy