I would recommend linux mint. Its completely free, has a nice gui, and is fairly straight forward to use. All you need to do is download the iso file from their website. If you want to try using it you can put it in a virtual machine to try it out first and then if you want to actually use it just download a program called rufus to flash the iso file onto a flash drive and then boot from the flash drive. There are plenty of tutorials on how to do this on youtube.