I need to add a forum to my website?????

For an existing website, you would want to find a forum that integrates with your platform. If you'd rather go the other way around, I'd suggest setup the forum first, then look into how to make the website integrate with it.

In example, once upon a time I had a forum and a website. We had a custom server control panel on the website and we didn't want to re-build an authentication system with role/permission management. At that time, we were using MyBB for our forum so, after several hours of googling, some trials and errors, we were able to integrate the main website login system with that of MyBB and our staff were able to effortlessly login, manage server punishments etc.