Free fast and secure Linux based operating system for everyone, suitable replacement to Windows or MacOS with different Desktop Environments
There are many different "distributions" of Linux. They all offer something a little different. Here are three distros that are popular among the community. You will be able to download a distribution of your choice at any of these sights.
Keep in mind there are far more than the three I just posted, but its hard to know which one you'll end up going with. Check out some of the distro specific threads on this sight to get a better idea of what you're looking for!
When you say "free" that hints that you expect like windows (genuine) there will be a charge. Well #Linux and a lot of programs are GNU licence free ;that means you can download with the lead /core team support for free with no looking over your shoulder. Most would appreciate though some financial support