Add Forum To Website

We are looking at adding a forum to our website [login to view URL] We would like the forum link to be located in the current header of the website and the design to match the current website (so the header is the same and then obviously the body will be different but same width and colour scheme). The other thing we would like is that members of our store can also use that login to access the forum (so by becoming a member of our store you are a member of the forum). The forum software also needs to be secure from hackers etc.