Linux refers to what is called an "operating system kernel," not a specific operating system. As there are many Linux distributions available, an individual must first determine which distribution of Linux best fits his needs. Afterwards, he can download that distribution from its official website.
As of 2015, Ubuntu is one of the most widely used Linux distributions and is available for download at Ubuntu.com. Linux Mint also enjoys significant popularity and should be easier for an ex-Windows user to adapt to due to the similarities in the interfaces of both operating systems. LinuxMint.com's download section provides links for each version of Linux Mint.