Hi and welcome to LQ
If you look to the right, you will see our distro review section - if you follow the link, you will see LQ member submitted reviews which should help you choose. You could also do a search of LQ for "best distro" or similar - you will see a huge nmber of discussions on this. Once you have chosen a distro, go to the "Download Linux" link to the right and grab the .iso file for your distro.
If you have never burned an iso disk, go to the LinuxQuestions portion of our Tutorials (link at top of page) and read up on how to burn your disk.
Good luck and we're here to help!