That's not exactly a specific question, so I'll just give a couple of pointers.
The world is full of people who thought that their forum would get busy in spite of the fact that the majority are less populated than the Sahara. Yours has to offer something a bit special in some way, have a sufficiently large potential user base, and be appropriately promoted. Building that user base can be a chicken-and-egg problem.
The only way to run a website at little or no cost (apart from your time) is to avoid the very traffic that you hope to attract so that you can stay within the bounds of a free host service. Google it and you will find plenty, but check the small print. Some are more philanthropic than others but most are likely to have tiny limits or advertising. With paid hosting available for c.£30 per annum, plus less than £10 for a domain name, it's hardly worth the saving.