GNS3 is extremely simple to install.
Either go to GNS3.com and sign up to receive a download link
Check out GNS3 on GitHub
But you are not done at all! As of now, the program is (mostly) USELESS.
GNS3 does not come with any Cisco IOS(router)/IOU(switch) firmware due to legal/licensing issues.
Please read guides for configuring the program further.
Sites to bookmark: