You can get the latest version of phpBB3 here: http://www.phpbb.com/downloads/olympus.php?from=submenu
But as LGforum said, you will need hosting. There are many free hosts around, and for the most part you get what you pay for.
However, they do serve a purpose...they will allow you to learn how to create and manage a website without having to pay for the site while you're learning. I've seen far too many people get their own shared-hosting site and have NO IDEA how to manage it. The phpBB and Wordpress support forums are littered with stupid
questions from such people. Please don't be one of them!
So my suggestion is to sign up for a free hosting plan, and learn how to manage the site. You can even download and install phpBB3, Wordpress, and phpMyAdmin (the latter two may not work on some free hosting sites due to their memory requirements). Once you feel comfortable with managing the site, you can sign up for a shared hosting plan -- many are available for under US$5/month -- and move your files/databases to the new site. NOTE
: I suggest that you NOT transfer a domain name to the free hosting site...many are like eToxic where you may be forced to wait months to transfer it to a new site.
I hope this answers your questions.