Well my friend, if you know what is the distro you want to get, you can go to the official site of that distribution, if not you can go to http://distrowatch.com or you can use this test to know wich system is for you http://www.zegeniestudios.net/ldc/ then download and try any distro.
Just to expand on the other comments of "Which one", as you seem a bit uninformed about Linux operating systems.
Linux is actually the kernel, this being the heart and soul of a computer and is what communicates with the hardware and software. On the top of this you have what are called distros, or distributions which are the Operating System. There are a huge number of Linux based distros which you can choose from, some being for specific purposes whilst others being for general use.
Ubuntu is the most popular Linux distro, and is designed for day to day desktop use. It has recently been updated to 13.04 although as you seem new to this, I would recommend 12.04 or 12.10 as they have less bugs.
Linux Mint is a second popular distro, again designed for desktop use but arguably a bit more user friendly.
Just to give an example of some advanced distributions, or those designed for specific tasks you also have things like Fedora which is based on the paid for Red Hat, an enterprise distribution.
A second example being Backtrack Linux which is designed for penetration testing.