Use http://www.prophpbb.com. It's free and already set up, but limited in terms of installing mods. Some come pre-installed and just need to be enabled in the Admin Control Panel, but if you feel the need to have a specific mod, you'll have to get a domain name and server hosting elsewhere.
I already have a website with buycraft is it possible to do it with buycraft? Or to somehow connect buycraft with enjin ?
As in you have a store on a Buycraft subdomain? (*storename*.buycraft.com or whatever it is)
If yes, no. In terms of connecting Buycraft with Enjin, most people do it via an iframe (or however it is done, I can't remember anymore) or just update the Buycraft theme to match the theme of their website/forum etc as closely as possible.
I hope the author solved the issue, but I was in the same boat several months ago and decided to share my experience. It is possible to use one of these free forum software. It will help you, especially in case you don't have much coding skills. i hope, someone will find it useful!