Just sign up and follow the instructions. The only thing you need is an email address. ( Just make a new gmail address if you don't want to use an existing one )
When you finish the first guided steps, you will have a username and password for your new empty forum and for an administration control panel. They host the forum for you, you do not need a web server or anything.
Somewhere on their website, there are videos on how to set up a forum step by step. It's been a long time, and I don't have the links anymore. If you make the free account and forum and poke around, it shouldn't be too hard to find. They also have a support forum that you can ask questions on.
As for how to customize the forum, there should be videos on how to do that too. The easiest way to do it would be to browse through their free collection of templates, choose one that you like and change the logo on it. ( again, there should be a video on how to do that too )
For what it's worth, it looks like someone posted these videos about 8 years aog. Given how little myfreeforums has changed over the years, they are probably still relevant. ( I sampled one - the audio was terrible, but it's better than nothing )
https://www.youtube.com/user/pdbravo/videos